MySQL/MariaDB数据库的多表查询操作
作者：尹正杰
版权声明：原创作品，谢绝转载！否则将追究法律责任。
一.单表查询小试牛刀
[root@node105.yinzhengjie.org.cn ~]# cat yinzhengjie_innodb.sql
-- MySQL dump 10.13 Distrib 5.5.33, for Linux (x86_64)
--
-- Host: localhost Database: yinzhengjie
-- ------------------------------------------------------
-- Server version 5.5.33-log /*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8 */;
/*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */;
/*!40103 SET TIME_ZONE='+00:00' */;
/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */; --
-- Current Database: `yinzhengjie`
-- CREATE DATABASE /*!32312 IF NOT EXISTS*/ `yinzhengjie` /*!40100 DEFAULT CHARACTER SET utf8 */; USE `yinzhengjie`; --
-- Table structure for table `classes`
-- DROP TABLE IF EXISTS `classes`;
/*!40101 SET @saved_cs_client = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `classes` (
`ClassID` tinyint(3) unsigned NOT NULL AUTO_INCREMENT,
`Class` varchar(100) DEFAULT NULL,
`NumOfStu` smallint(5) unsigned DEFAULT NULL,
PRIMARY KEY (`ClassID`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */; --
-- Dumping data for table `classes`
-- LOCK TABLES `classes` WRITE;
/*!40000 ALTER TABLE `classes` DISABLE KEYS */;
INSERT INTO `classes` VALUES (1,'Shaolin Pai',10),(2,'Emei Pai',7),(3,'QingCheng Pai',11),(4,'Wudang Pai',12),(5,'Riyue Shenjiao',31),(6,'Liansha
n Pai',27),(7,'Ming Jiao',27),(8,'Xiaoyao Pai',15);/*!40000 ALTER TABLE `classes` ENABLE KEYS */;
UNLOCK TABLES; --
-- Table structure for table `coc`
-- DROP TABLE IF EXISTS `coc`;
/*!40101 SET @saved_cs_client = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `coc` (
`ID` int(10) unsigned NOT NULL AUTO_INCREMENT,
`ClassID` tinyint(3) unsigned NOT NULL,
`CourseID` smallint(5) unsigned DEFAULT NULL,
PRIMARY KEY (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */; --
-- Dumping data for table `coc`
-- LOCK TABLES `coc` WRITE;
/*!40000 ALTER TABLE `coc` DISABLE KEYS */;
INSERT INTO `coc` VALUES (1,1,2),(2,1,5),(3,2,2),(4,2,6),(5,3,1),(6,3,7),(7,4,5),(8,4,2),(9,5,1),(10,5,9),(11,6,3),(12,6,4),(13,7,4),(14,7,3);
/*!40000 ALTER TABLE `coc` ENABLE KEYS */;
UNLOCK TABLES; --
-- Table structure for table `courses`
-- DROP TABLE IF EXISTS `courses`;
/*!40101 SET @saved_cs_client = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `courses` (
`CourseID` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`Course` varchar(100) NOT NULL,
PRIMARY KEY (`CourseID`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */; --
-- Dumping data for table `courses`
-- LOCK TABLES `courses` WRITE;
/*!40000 ALTER TABLE `courses` DISABLE KEYS */;
INSERT INTO `courses` VALUES (1,'Hamo Gong'),(2,'Kuihua Baodian'),(3,'Jinshe Jianfa'),(4,'Taiji Quan'),(5,'Daiyu Zanghua'),(6,'Weituo Zhang'),(7,
'Dagou Bangfa');/*!40000 ALTER TABLE `courses` ENABLE KEYS */;
UNLOCK TABLES; --
-- Table structure for table `scores`
-- DROP TABLE IF EXISTS `scores`;
/*!40101 SET @saved_cs_client = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `scores` (
`ID` int(10) unsigned NOT NULL AUTO_INCREMENT,
`StuID` int(10) unsigned NOT NULL,
`CourseID` smallint(5) unsigned NOT NULL,
`Score` tinyint(3) unsigned DEFAULT NULL,
PRIMARY KEY (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=16 DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */; --
-- Dumping data for table `scores`
-- LOCK TABLES `scores` WRITE;
/*!40000 ALTER TABLE `scores` DISABLE KEYS */;
INSERT INTO `scores` VALUES (1,1,2,77),(2,1,6,93),(3,2,2,47),(4,2,5,97),(5,3,2,88),(6,3,6,75),(7,4,5,71),(8,4,2,89),(9,5,1,39),(10,5,7,63),(11,6,
1,96),(12,7,1,86),(13,7,7,83),(14,8,4,57),(15,8,3,93);/*!40000 ALTER TABLE `scores` ENABLE KEYS */;
UNLOCK TABLES; --
-- Table structure for table `students`
-- DROP TABLE IF EXISTS `students`;
/*!40101 SET @saved_cs_client = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `students` (
`StuID` int(10) unsigned NOT NULL AUTO_INCREMENT,
`Name` varchar(50) NOT NULL,
`Age` tinyint(3) unsigned NOT NULL,
`Gender` enum('F','M') NOT NULL,
`ClassID` tinyint(3) unsigned DEFAULT NULL,
`TeacherID` int(10) unsigned DEFAULT NULL,
PRIMARY KEY (`StuID`)
) ENGINE=InnoDB AUTO_INCREMENT=26 DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */; --
-- Dumping data for table `students`
-- LOCK TABLES `students` WRITE;
/*!40000 ALTER TABLE `students` DISABLE KEYS */;
INSERT INTO `students` VALUES (1,'Shi Zhongyu',22,'M',2,3),(2,'Shi Potian',22,'M',1,7),(3,'Xie Yanke',53,'M',2,16),(4,'Ding Dian',32,'M',4,4),(5,
'Yu Yutong',26,'M',3,1),(6,'Shi Qing',46,'M',5,NULL),(7,'Xi Ren',19,'F',3,NULL),(8,'Lin Daiyu',17,'F',7,NULL),(9,'Ren Yingying',20,'F',6,NULL),(10,'Yue Lingshan',19,'F',3,NULL),(11,'Yuan Chengzhi',23,'M',6,NULL),(12,'Wen Qingqing',19,'F',1,NULL),(13,'Tian Boguang',33,'M',2,NULL),(14,'Lu Wushuang',17,'F',3,NULL),(15,'Duan Yu',19,'M',4,NULL),(16,'Xu Zhu',21,'M',1,NULL),(17,'Lin Chong',25,'M',4,NULL),(18,'Hua Rong',23,'M',7,NULL),(19,'Xue Baochai',18,'F',6,NULL),(20,'Diao Chan',19,'F',7,NULL),(21,'Huang Yueying',22,'F',6,NULL),(22,'Xiao Qiao',20,'F',1,NULL),(23,'Ma Chao',23,'M',4,NULL),(24,'Xu Xian',27,'M',NULL,NULL),(25,'Sun Dasheng',100,'M',NULL,NULL);/*!40000 ALTER TABLE `students` ENABLE KEYS */;
UNLOCK TABLES; --
-- Table structure for table `teachers`
-- DROP TABLE IF EXISTS `teachers`;
/*!40101 SET @saved_cs_client = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `teachers` (
`TID` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`Name` varchar(100) NOT NULL,
`Age` tinyint(3) unsigned NOT NULL,
`Gender` enum('F','M') DEFAULT NULL,
PRIMARY KEY (`TID`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */; --
-- Dumping data for table `teachers`
-- LOCK TABLES `teachers` WRITE;
/*!40000 ALTER TABLE `teachers` DISABLE KEYS */;
INSERT INTO `teachers` VALUES (1,'Song Jiang',45,'M'),(2,'Zhang Sanfeng',94,'M'),(3,'Miejue Shitai',77,'F'),(4,'Lin Chaoying',93,'F');
/*!40000 ALTER TABLE `teachers` ENABLE KEYS */;
UNLOCK TABLES; --
-- Table structure for table `toc`
-- DROP TABLE IF EXISTS `toc`;
/*!40101 SET @saved_cs_client = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `toc` (
`ID` int(10) unsigned NOT NULL AUTO_INCREMENT,
`CourseID` smallint(5) unsigned DEFAULT NULL,
`TID` smallint(5) unsigned DEFAULT NULL,
PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */; --
-- Dumping data for table `toc`
-- LOCK TABLES `toc` WRITE;
/*!40000 ALTER TABLE `toc` DISABLE KEYS */;
/*!40000 ALTER TABLE `toc` ENABLE KEYS */;
UNLOCK TABLES;
/*!40103 SET TIME_ZONE=@OLD_TIME_ZONE */; /*!40101 SET SQL_MODE=@OLD_SQL_MODE */;
/*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */;
/*!40014 SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS */;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;
/*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */; -- Dump completed on 2013-09-03 2:51:27
[root@node105.yinzhengjie.org.cn ~]#
[root@node105.yinzhengjie.org.cn ~]# cat yinzhengjie_innodb.sql
导入yinzhengjie_innodb.sql生成数据库及相应的测试表数据信息，然后完成下面问题。
[root@node105.yinzhengjie.org.cn ~]# mysql -uroot -pyinzhengjie -S /mysql/3306/socket/mysql.sock
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 17
Server version: 10.2.19-MariaDB MariaDB Server Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. MariaDB [(none)]>
MariaDB [(none)]> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
+--------------------+
rows in set (0.00 sec) MariaDB [(none)]>
MariaDB [(none)]> SOURCE yinzhengjie_innodb.sql
Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 1 row affected (0.00 sec) Database changed
Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 8 rows affected (0.01 sec)
Records: 8 Duplicates: 0 Warnings: 0 Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 14 rows affected (0.00 sec)
Records: 14 Duplicates: 0 Warnings: 0 Query OK, 0 rows affected (0.01 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 7 rows affected (0.00 sec)
Records: 7 Duplicates: 0 Warnings: 0 Query OK, 0 rows affected (0.01 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 15 rows affected (0.00 sec)
Records: 15 Duplicates: 0 Warnings: 0 Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.01 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 25 rows affected (0.00 sec)
Records: 25 Duplicates: 0 Warnings: 0 Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.01 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0 Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SHOW DATABASES;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| yinzhengjie |
+--------------------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SHOW TABLES;
+-----------------------+
| Tables_in_yinzhengjie |
+-----------------------+
| classes |
| coc |
| courses |
| scores |
| students |
| teachers |
| toc |
+-----------------------+
7 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]>
MariaDB [(none)]> SOURCE yinzhengjie_innodb.sql
1>.在students表中，查询年龄大于25岁，且为男性的同学的名字和年龄
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT Name AS 姓名, Age AS 年龄 FROM students WHERE Age > 25 AND Gender = 'M';
+--------------+--------+
| 姓名 | 年龄 |
+--------------+--------+
| Xie Yanke | 53 |
| Ding Dian | 32 |
| Yu Yutong | 26 |
| Shi Qing | 46 |
| Tian Boguang | 33 |
| Xu Xian | 27 |
| Sun Dasheng | 100 |
+--------------+--------+
7 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT Name AS 姓名, Age AS 年龄 FROM students WHERE Age > 25 AND Gender = 'M';
2>.以ClassID为分组依据，显示每组的平均年龄
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT ClassID,AVG(Age) FROM students GROUP BY ClassID; #查询的结果NULL那一行始终在最上面
+---------+----------+
| ClassID | AVG(Age) |
+---------+----------+
| NULL | 63.5000 |
| 1 | 20.5000 |
| 2 | 36.0000 |
| 3 | 20.2500 |
| 4 | 24.7500 |
| 5 | 46.0000 |
| 6 | 20.7500 |
| 7 | 19.6667 |
+---------+----------+
8 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT ClassID,AVG(Age) FROM students GROUP BY -ClassID DESC; #可以将NULL的结果放在最后一行输出
+---------+----------+
| ClassID | AVG(Age) |
+---------+----------+
| 1 | 20.5000 |
| 2 | 36.0000 |
| 3 | 20.2500 |
| 4 | 24.7500 |
| 5 | 46.0000 |
| 6 | 20.7500 |
| 7 | 19.6667 |
| NULL | 63.5000 |
+---------+----------+
8 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT ClassID,AVG(Age) FROM students GROUP BY -ClassID DESC; #可以将NULL的结果放在最后一行输出
3>.显示第2题中平均年龄大于30的分组及平均年龄
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT ClassID,AVG(Age) FROM students GROUP BY -ClassID DESC;
+---------+----------+
| ClassID | AVG(Age) |
+---------+----------+
| 1 | 20.5000 |
| 2 | 36.0000 |
| 3 | 20.2500 |
| 4 | 24.7500 |
| 5 | 46.0000 |
| 6 | 20.7500 |
| 7 | 19.6667 |
| NULL | 63.5000 |
+---------+----------+
8 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT ClassID,AVG(Age) AS age FROM students GROUP BY -ClassID DESC HAVING age >30;
+---------+---------+
| ClassID | age |
+---------+---------+
| 2 | 36.0000 |
| 5 | 46.0000 |
| NULL | 63.5000 |
+---------+---------+
3 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT ClassID,AVG(Age) AS age FROM students GROUP BY -ClassID DESC HAVING age >30;
4>.显示以L开头的名字的同学的信息
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students WHERE Name RLIKE '^L';
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
+-------+-------------+-----+--------+---------+-----------+
3 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students WHERE Name RLIKE '^L';
5>.显示TeacherID非空的同学的相关信息
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students WHERE TeacherID IS NOT NULL;
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
+-------+-------------+-----+--------+---------+-----------+
5 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students WHERE TeacherID IS NOT NULL;
6>.以年龄排序后，显示年龄最大的前10位同学的信息
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students ORDER BY Age DESC LIMIT 10;
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
+-------+--------------+-----+--------+---------+-----------+
10 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students ORDER BY Age DESC LIMIT 10;
7>.查询年龄大于等于20岁，小于等于25岁的同学的信息
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]> SELECT * FROM students WHERE Age BETWEEN 20 AND 25;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
+-------+---------------+-----+--------+---------+-----------+
10 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students WHERE Age BETWEEN 20 AND 25;
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students WHERE Age >= 20 AND AGE <= 25;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
+-------+---------------+-----+--------+---------+-----------+
10 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students WHERE Age >= 20 AND AGE <= 25;
二.多表查询(基于上面导入的"yinzhengjie_innodb.sql"数据进行演示)
博主推荐阅读:
https://www.runoob.com/sql/sql-join.html
1>.将两张表中个别字段联合显示(纵向合并)
联合查询：UNION
拥有自动去重的功能 两张表中字段数量得相同 示例:
SELECT Name,Age FROM students UNION SELECT Name,Age FROM teachers;
MariaDB [yinzhengjie]> SHOW TABLES;
+-----------------------+
| Tables_in_yinzhengjie |
+-----------------------+
| classes |
| coc |
| courses |
| scores |
| students |
| teachers |
| toc |
+-----------------------+
7 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT StuID AS id ,Name AS 姓名 FROM students UNION SELECT TID,Name FROM teachers;
+----+---------------+
| id | 姓名 |
+----+---------------+
| 1 | Shi Zhongyu |
| 2 | Shi Potian |
| 3 | Xie Yanke |
| 4 | Ding Dian |
| 5 | Yu Yutong |
| 6 | Shi Qing |
| 7 | Xi Ren |
| 8 | Lin Daiyu |
| 9 | Ren Yingying |
| 10 | Yue Lingshan |
| 11 | Yuan Chengzhi |
| 12 | Wen Qingqing |
| 13 | Tian Boguang |
| 14 | Lu Wushuang |
| 15 | Duan Yu |
| 16 | Xu Zhu |
| 17 | Lin Chong |
| 18 | Hua Rong |
| 19 | Xue Baochai |
| 20 | Diao Chan |
| 21 | Huang Yueying |
| 22 | Xiao Qiao |
| 23 | Ma Chao |
| 24 | Xu Xian |
| 25 | Sun Dasheng |
| 1 | Song Jiang |
| 2 | Zhang Sanfeng |
| 3 | Miejue Shitai |
| 4 | Lin Chaoying |
+----+---------------+
29 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT StuID AS id ,Name AS 姓名 FROM students UNION SELECT TID,Name FROM teachers;(UNION会自动去重)
2>.笛卡尔乘积查询(又称交叉连接，横向合并，生成环境应该避免这种查询)
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students CROSS JOIN teachers; #笛卡尔积查询，生产环境应该避免这样的查询。
+-------+---------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+---------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 1 | Song Jiang | 45 | M |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 2 | Zhang Sanfeng | 94 | M |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 3 | Miejue Shitai | 77 | F |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 4 | Lin Chaoying | 93 | F |
| 2 | Shi Potian | 22 | M | 1 | 7 | 1 | Song Jiang | 45 | M |
| 2 | Shi Potian | 22 | M | 1 | 7 | 2 | Zhang Sanfeng | 94 | M |
| 2 | Shi Potian | 22 | M | 1 | 7 | 3 | Miejue Shitai | 77 | F |
| 2 | Shi Potian | 22 | M | 1 | 7 | 4 | Lin Chaoying | 93 | F |
| 3 | Xie Yanke | 53 | M | 2 | 16 | 1 | Song Jiang | 45 | M |
| 3 | Xie Yanke | 53 | M | 2 | 16 | 2 | Zhang Sanfeng | 94 | M |
| 3 | Xie Yanke | 53 | M | 2 | 16 | 3 | Miejue Shitai | 77 | F |
| 3 | Xie Yanke | 53 | M | 2 | 16 | 4 | Lin Chaoying | 93 | F |
| 4 | Ding Dian | 32 | M | 4 | 4 | 1 | Song Jiang | 45 | M |
| 4 | Ding Dian | 32 | M | 4 | 4 | 2 | Zhang Sanfeng | 94 | M |
| 4 | Ding Dian | 32 | M | 4 | 4 | 3 | Miejue Shitai | 77 | F |
| 4 | Ding Dian | 32 | M | 4 | 4 | 4 | Lin Chaoying | 93 | F |
| 5 | Yu Yutong | 26 | M | 3 | 1 | 1 | Song Jiang | 45 | M |
| 5 | Yu Yutong | 26 | M | 3 | 1 | 2 | Zhang Sanfeng | 94 | M |
| 5 | Yu Yutong | 26 | M | 3 | 1 | 3 | Miejue Shitai | 77 | F |
| 5 | Yu Yutong | 26 | M | 3 | 1 | 4 | Lin Chaoying | 93 | F |
| 6 | Shi Qing | 46 | M | 5 | NULL | 1 | Song Jiang | 45 | M |
| 6 | Shi Qing | 46 | M | 5 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 6 | Shi Qing | 46 | M | 5 | NULL | 3 | Miejue Shitai | 77 | F |
| 6 | Shi Qing | 46 | M | 5 | NULL | 4 | Lin Chaoying | 93 | F |
| 7 | Xi Ren | 19 | F | 3 | NULL | 1 | Song Jiang | 45 | M |
| 7 | Xi Ren | 19 | F | 3 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 7 | Xi Ren | 19 | F | 3 | NULL | 3 | Miejue Shitai | 77 | F |
| 7 | Xi Ren | 19 | F | 3 | NULL | 4 | Lin Chaoying | 93 | F |
| 8 | Lin Daiyu | 17 | F | 7 | NULL | 1 | Song Jiang | 45 | M |
| 8 | Lin Daiyu | 17 | F | 7 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 8 | Lin Daiyu | 17 | F | 7 | NULL | 3 | Miejue Shitai | 77 | F |
| 8 | Lin Daiyu | 17 | F | 7 | NULL | 4 | Lin Chaoying | 93 | F |
| 9 | Ren Yingying | 20 | F | 6 | NULL | 1 | Song Jiang | 45 | M |
| 9 | Ren Yingying | 20 | F | 6 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 9 | Ren Yingying | 20 | F | 6 | NULL | 3 | Miejue Shitai | 77 | F |
| 9 | Ren Yingying | 20 | F | 6 | NULL | 4 | Lin Chaoying | 93 | F |
| 10 | Yue Lingshan | 19 | F | 3 | NULL | 1 | Song Jiang | 45 | M |
| 10 | Yue Lingshan | 19 | F | 3 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 10 | Yue Lingshan | 19 | F | 3 | NULL | 3 | Miejue Shitai | 77 | F |
| 10 | Yue Lingshan | 19 | F | 3 | NULL | 4 | Lin Chaoying | 93 | F |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL | 1 | Song Jiang | 45 | M |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL | 3 | Miejue Shitai | 77 | F |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL | 4 | Lin Chaoying | 93 | F |
| 12 | Wen Qingqing | 19 | F | 1 | NULL | 1 | Song Jiang | 45 | M |
| 12 | Wen Qingqing | 19 | F | 1 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 12 | Wen Qingqing | 19 | F | 1 | NULL | 3 | Miejue Shitai | 77 | F |
| 12 | Wen Qingqing | 19 | F | 1 | NULL | 4 | Lin Chaoying | 93 | F |
| 13 | Tian Boguang | 33 | M | 2 | NULL | 1 | Song Jiang | 45 | M |
| 13 | Tian Boguang | 33 | M | 2 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 13 | Tian Boguang | 33 | M | 2 | NULL | 3 | Miejue Shitai | 77 | F |
| 13 | Tian Boguang | 33 | M | 2 | NULL | 4 | Lin Chaoying | 93 | F |
| 14 | Lu Wushuang | 17 | F | 3 | NULL | 1 | Song Jiang | 45 | M |
| 14 | Lu Wushuang | 17 | F | 3 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 14 | Lu Wushuang | 17 | F | 3 | NULL | 3 | Miejue Shitai | 77 | F |
| 14 | Lu Wushuang | 17 | F | 3 | NULL | 4 | Lin Chaoying | 93 | F |
| 15 | Duan Yu | 19 | M | 4 | NULL | 1 | Song Jiang | 45 | M |
| 15 | Duan Yu | 19 | M | 4 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 15 | Duan Yu | 19 | M | 4 | NULL | 3 | Miejue Shitai | 77 | F |
| 15 | Duan Yu | 19 | M | 4 | NULL | 4 | Lin Chaoying | 93 | F |
| 16 | Xu Zhu | 21 | M | 1 | NULL | 1 | Song Jiang | 45 | M |
| 16 | Xu Zhu | 21 | M | 1 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 16 | Xu Zhu | 21 | M | 1 | NULL | 3 | Miejue Shitai | 77 | F |
| 16 | Xu Zhu | 21 | M | 1 | NULL | 4 | Lin Chaoying | 93 | F |
| 17 | Lin Chong | 25 | M | 4 | NULL | 1 | Song Jiang | 45 | M |
| 17 | Lin Chong | 25 | M | 4 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 17 | Lin Chong | 25 | M | 4 | NULL | 3 | Miejue Shitai | 77 | F |
| 17 | Lin Chong | 25 | M | 4 | NULL | 4 | Lin Chaoying | 93 | F |
| 18 | Hua Rong | 23 | M | 7 | NULL | 1 | Song Jiang | 45 | M |
| 18 | Hua Rong | 23 | M | 7 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 18 | Hua Rong | 23 | M | 7 | NULL | 3 | Miejue Shitai | 77 | F |
| 18 | Hua Rong | 23 | M | 7 | NULL | 4 | Lin Chaoying | 93 | F |
| 19 | Xue Baochai | 18 | F | 6 | NULL | 1 | Song Jiang | 45 | M |
| 19 | Xue Baochai | 18 | F | 6 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 19 | Xue Baochai | 18 | F | 6 | NULL | 3 | Miejue Shitai | 77 | F |
| 19 | Xue Baochai | 18 | F | 6 | NULL | 4 | Lin Chaoying | 93 | F |
| 20 | Diao Chan | 19 | F | 7 | NULL | 1 | Song Jiang | 45 | M |
| 20 | Diao Chan | 19 | F | 7 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 20 | Diao Chan | 19 | F | 7 | NULL | 3 | Miejue Shitai | 77 | F |
| 20 | Diao Chan | 19 | F | 7 | NULL | 4 | Lin Chaoying | 93 | F |
| 21 | Huang Yueying | 22 | F | 6 | NULL | 1 | Song Jiang | 45 | M |
| 21 | Huang Yueying | 22 | F | 6 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 21 | Huang Yueying | 22 | F | 6 | NULL | 3 | Miejue Shitai | 77 | F |
| 21 | Huang Yueying | 22 | F | 6 | NULL | 4 | Lin Chaoying | 93 | F |
| 22 | Xiao Qiao | 20 | F | 1 | NULL | 1 | Song Jiang | 45 | M |
| 22 | Xiao Qiao | 20 | F | 1 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 22 | Xiao Qiao | 20 | F | 1 | NULL | 3 | Miejue Shitai | 77 | F |
| 22 | Xiao Qiao | 20 | F | 1 | NULL | 4 | Lin Chaoying | 93 | F |
| 23 | Ma Chao | 23 | M | 4 | NULL | 1 | Song Jiang | 45 | M |
| 23 | Ma Chao | 23 | M | 4 | NULL | 2 | Zhang Sanfeng | 94 | M |
| 23 | Ma Chao | 23 | M | 4 | NULL | 3 | Miejue Shitai | 77 | F |
| 23 | Ma Chao | 23 | M | 4 | NULL | 4 | Lin Chaoying | 93 | F |
| 24 | Xu Xian | 27 | M | NULL | NULL | 1 | Song Jiang | 45 | M |
| 24 | Xu Xian | 27 | M | NULL | NULL | 2 | Zhang Sanfeng | 94 | M |
| 24 | Xu Xian | 27 | M | NULL | NULL | 3 | Miejue Shitai | 77 | F |
| 24 | Xu Xian | 27 | M | NULL | NULL | 4 | Lin Chaoying | 93 | F |
| 25 | Sun Dasheng | 100 | M | NULL | NULL | 1 | Song Jiang | 45 | M |
| 25 | Sun Dasheng | 100 | M | NULL | NULL | 2 | Zhang Sanfeng | 94 | M |
| 25 | Sun Dasheng | 100 | M | NULL | NULL | 3 | Miejue Shitai | 77 | F |
| 25 | Sun Dasheng | 100 | M | NULL | NULL | 4 | Lin Chaoying | 93 | F |
+-------+---------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
100 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students CROSS JOIN teachers; #笛卡尔积查询，生产环境应该避免这样的查询。
3>.内连接查询
等值连接：让表之间的字段以“等值”建立连接关系；
不等值连接 自然连接:去掉重复列的等值连接 自连接
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students,teachers WHERE students.TeacherID = teachers.TID; #该写法太老，不推荐使用
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| 5 | Yu Yutong | 26 | M | 3 | 1 | 1 | Song Jiang | 45 | M |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 3 | Miejue Shitai | 77 | F |
| 4 | Ding Dian | 32 | M | 4 | 4 | 4 | Lin Chaoying | 93 | F |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
3 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students,teachers WHERE students.TeacherID = teachers.TID; #该写法太老，不推荐使用
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students INNER JOIN teachers ON students.TeacherID = teachers.TID; #内连接查询，推荐使用该写法
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| 5 | Yu Yutong | 26 | M | 3 | 1 | 1 | Song Jiang | 45 | M |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 3 | Miejue Shitai | 77 | F |
| 4 | Ding Dian | 32 | M | 4 | 4 | 4 | Lin Chaoying | 93 | F |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
3 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students INNER JOIN teachers ON students.TeacherID = teachers.TID; #内连接查询，推荐使用该写法
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT stuid,s.name AS 学生姓名 ,tid,t.name AS 老师姓名 FROM students AS s INNER JOIN teachers AS t ON s.TeacherID = t.TID; #查询制定者字段并设置字段别名,也为表名设置别名进行调用。
+-------+--------------+-----+---------------+
| stuid | 学生姓名 | tid | 老师姓名 |
+-------+--------------+-----+---------------+
| 5 | Yu Yutong | 1 | Song Jiang |
| 1 | Shi Zhongyu | 3 | Miejue Shitai |
| 4 | Ding Dian | 4 | Lin Chaoying |
+-------+--------------+-----+---------------+
3 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT stuid,s.name AS 学生姓名 ,tid,t.name AS 老师姓名 FROM students AS s INNER JOIN teachers AS t ON s.TeacherID = t.TID; #查询制定者字段并设置字段别名,也为表名设置别名进行调用。(一旦为表设置别名就不能在调用该表原来的名称)
MariaDB [yinzhengjie]> CREATE TABLE emp(id int,name char(3),leaderid int);
Query OK, 0 rows affected (0.01 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> DESC emp;
+----------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | char(3) | YES | | NULL | |
| leaderid | int(11) | YES | | NULL | |
+----------+---------+------+-----+---------+-------+
rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> INSERT INTO emp VALUES(1,'张无忌',null),(2,'杨逍',1),(3,'范遥',2),(4,'殷天正',2),(5,'韦一笑',2);
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0 MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM emp;
+------+-----------+----------+
| id | name | leaderid |
+------+-----------+----------+
| 1 | 张无忌 | NULL |
| 2 | 杨逍 | 1 |
| 3 | 范遥 | 2 |
| 4 | 殷天正 | 2 |
| 5 | 韦一笑 | 2 |
+------+-----------+----------+
rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT a.name,b.name AS 领导 FROM emp AS a LEFT OUTER JOIN emp AS b ON a.leaderid = b.id; #同一张表查询数据，查询每个人及其领导的姓名。(自连接查询)
+-----------+-----------+
| name | 领导 |
+-----------+-----------+
| 杨逍 | 张无忌 |
| 范遥 | 杨逍 |
| 殷天正 | 杨逍 |
| 韦一笑 | 杨逍 |
| 张无忌 | NULL |
+-----------+-----------+
rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT a.name,b.name AS 领导 FROM emp AS a LEFT OUTER JOIN emp AS b ON a.leaderid = b.id; #同一张表查询数据，查询每个人及其领导的姓名。(自连接查询)
4>.子查询
子查询：在查询语句嵌套着查询语句，性能较差
基于某语句的查询结果再次进行的查询
用在WHERE子句中的子查询
用于比较表达式中的子查询；子查询仅能返回单个值
SELECT Name,Age FROM students WHERE Age>(SELECT avg(Age) FROM students);
用于IN中的子查询：子查询应该单键查询并返回一个或多个值从构成列表
SELECT Name,Age FROM students WHERE Age IN (SELECT Age FROM teachers);
用于EXISTS
用于FROM子句中的子查询
使用格式：SELECT tb_alias.col1,... FROM (SELECT clause) AS tb_alias WHERE Clause;
示例：
SELECT s.aage,s.ClassID FROM (SELECT avg(Age) AS aage,ClassID FROM students WHERE ClassID IS NOT NULL GROUP BY ClassID) AS s WHERE s.aage>30;
MariaDB [yinzhengjie]> SELECT AVG(age) FROM students;
+----------+
| AVG(age) |
+----------+
| 27.4000 |
+----------+
row in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM teachers WHERE age > (SELECT AVG(age) FROM students) * 2;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM teachers WHERE age > (SELECT AVG(age) FROM students) * 2; #查询老师年龄大于学生平均年龄的2倍的人
5>.外连接查询
左外连接：
FROM tb1 LEFT JOIN tb2 ON tb1.col=tb2.col
右外连接
FROM tb1 RIGHT JOIN tb2 ON tb1.col=tb2.col
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s LEFT OUTER JOIN teachers AS t ON s.TeacherID = t.TID; #左外连接，以students表(在左边)记录为校验
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 3 | Miejue Shitai | 77 | F |
| 2 | Shi Potian | 22 | M | 1 | 7 | NULL | NULL | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 | NULL | NULL | NULL | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 | 4 | Lin Chaoying | 93 | F |
| 5 | Yu Yutong | 26 | M | 3 | 1 | 1 | Song Jiang | 45 | M |
| 6 | Shi Qing | 46 | M | 5 | NULL | NULL | NULL | NULL | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL | NULL | NULL | NULL | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL | NULL | NULL | NULL | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL | NULL | NULL | NULL | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL | NULL | NULL | NULL | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL | NULL | NULL | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL | NULL | NULL | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s LEFT OUTER JOIN teachers AS t ON s.TeacherID = t.TID; #左外连接，以students表(在左边)记录为校验
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s RIGHT OUTER JOIN teachers AS t ON s.TeacherID = t.TID; #右外连接,以"teachers"(在右边)表记录为校验
+-------+-------------+------+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+-------------+------+--------+---------+-----------+-----+---------------+-----+--------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 3 | Miejue Shitai | 77 | F |
| 4 | Ding Dian | 32 | M | 4 | 4 | 4 | Lin Chaoying | 93 | F |
| 5 | Yu Yutong | 26 | M | 3 | 1 | 1 | Song Jiang | 45 | M |
| NULL | NULL | NULL | NULL | NULL | NULL | 2 | Zhang Sanfeng | 94 | M |
+-------+-------------+------+--------+---------+-----------+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s RIGHT OUTER JOIN teachers AS t ON s.TeacherID = t.TID; #右外连接,以"teachers"(在右边)表记录为校验
MariaDB [yinzhengjie]> SELECT * FROM students AS s LEFT OUTER JOIN teachers AS t ON s.TeacherID = t.TID;
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 3 | Miejue Shitai | 77 | F |
| 2 | Shi Potian | 22 | M | 1 | 7 | NULL | NULL | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 | NULL | NULL | NULL | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 | 4 | Lin Chaoying | 93 | F |
| 5 | Yu Yutong | 26 | M | 3 | 1 | 1 | Song Jiang | 45 | M |
| 6 | Shi Qing | 46 | M | 5 | NULL | NULL | NULL | NULL | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL | NULL | NULL | NULL | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL | NULL | NULL | NULL | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL | NULL | NULL | NULL | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL | NULL | NULL | NULL | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL | NULL | NULL | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL | NULL | NULL | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s LEFT OUTER JOIN teachers AS t ON s.TeacherID = t.TID WHERE t.tid IS NULL; #对左外连接再次进行过滤，将两个表中重复的数据剔除。
+-------+---------------+-----+--------+---------+-----------+------+------+------+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+---------------+-----+--------+---------+-----------+------+------+------+--------+
| 2 | Shi Potian | 22 | M | 1 | 7 | NULL | NULL | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 | NULL | NULL | NULL | NULL |
| 6 | Shi Qing | 46 | M | 5 | NULL | NULL | NULL | NULL | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL | NULL | NULL | NULL | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL | NULL | NULL | NULL | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL | NULL | NULL | NULL | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL | NULL | NULL | NULL | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL | NULL | NULL | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL | NULL | NULL | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+------+------+------+--------+
22 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s LEFT OUTER JOIN teachers AS t ON s.TeacherID = t.TID WHERE t.tid IS NULL; #对左外连接再次进行过滤，将两个表中重复的数据剔除。(左外连接特例)
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s RIGHT OUTER JOIN teachers AS t ON s.TeacherID = t.TID;
+-------+-------------+------+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+-------------+------+--------+---------+-----------+-----+---------------+-----+--------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 3 | Miejue Shitai | 77 | F |
| 4 | Ding Dian | 32 | M | 4 | 4 | 4 | Lin Chaoying | 93 | F |
| 5 | Yu Yutong | 26 | M | 3 | 1 | 1 | Song Jiang | 45 | M |
| NULL | NULL | NULL | NULL | NULL | NULL | 2 | Zhang Sanfeng | 94 | M |
+-------+-------------+------+--------+---------+-----------+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s RIGHT OUTER JOIN teachers AS t ON s.TeacherID = t.TID WHERE s.teacherid IS NULL; #对右外连接再次进行过滤，将重复的数据剔除掉。
+-------+------+------+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+------+------+--------+---------+-----------+-----+---------------+-----+--------+
| NULL | NULL | NULL | NULL | NULL | NULL | 2 | Zhang Sanfeng | 94 | M |
+-------+------+------+--------+---------+-----------+-----+---------------+-----+--------+
1 row in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s RIGHT OUTER JOIN teachers AS t ON s.TeacherID = t.TID WHERE s.teacherid IS NULL; #对右外连接再次进行过滤，将重复的数据剔除掉。
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s FULL OUTER JOIN teachers AS t ON s.teacherid = t.tid; #MySQL/MariaDB不支持完全外连接查询
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax t
o use near 'FULL OUTER JOIN teachers AS t ON s.teacherid = t.tid' at line 1
MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s LEFT OUTER JOIN teachers AS t ON s.TeacherID = t.TID UNION SELECT * FROM students AS s RIGHT O
UTER JOIN teachers AS t ON s.TeacherID = t.TID; #由于MySQL/MariaDB不支持完全外连接查询，但是我们可以使用左外连接与右外连接的查询结果通过UNION去重功能实现完全外连接查询功能。
+-------+---------------+------+--------+---------+-----------+------+---------------+------+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+---------------+------+--------+---------+-----------+------+---------------+------+--------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 3 | Miejue Shitai | 77 | F |
| 2 | Shi Potian | 22 | M | 1 | 7 | NULL | NULL | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 | NULL | NULL | NULL | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 | 4 | Lin Chaoying | 93 | F |
| 5 | Yu Yutong | 26 | M | 3 | 1 | 1 | Song Jiang | 45 | M |
| 6 | Shi Qing | 46 | M | 5 | NULL | NULL | NULL | NULL | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL | NULL | NULL | NULL | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL | NULL | NULL | NULL | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL | NULL | NULL | NULL | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL | NULL | NULL | NULL | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL | NULL | NULL | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL | NULL | NULL | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | NULL | 2 | Zhang Sanfeng | 94 | M |
+-------+---------------+------+--------+---------+-----------+------+---------------+------+--------+
26 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students AS s LEFT OUTER JOIN teachers AS t ON s.TeacherID = t.TID UNION SELECT * FROM students AS s RIGHT O UTER JOIN teachers AS t ON s.TeacherID = t.TID; #由于MySQL/MariaDB不支持完全外连接查询，但是我们可以使用左外连接与右外连接的查询结果通过UNION去重功能实现完全外连接查询功能。
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM (SELECT stuid,s.name AS student_name,s.age AS student_age,s.gender AS student_gender,classid,teacherid,tid,t.name AS teacher_name,t.age AS teacher_age,t.gender AS teacher_gender FROM students AS s LEFT OUTER JOIN teachers AS t ON s.TeacherID = t.TID UNION SELECT stuid,s.name AS student_name,s.age AS student_age,s.gender AS student_gender,classid,teacherid,tid,t.name AS teacher_name,t.age AS teacher_age,t.gender AS teacher_gender FROM students AS s RIGHT OUTER JOIN teachers AS t ON s.TeacherID = t.TID) AS f WHERE f.teacherid IS NULL OR f.tid IS NULL; #将间接实现完全外连接查询结果再次进行过滤,查询出完全外连接中过滤掉两表重复的数据。
+-------+---------------+-------------+----------------+---------+-----------+------+---------------+-------------+----------------+
| stuid | student_name | student_age | student_gender | classid | teacherid | tid | teacher_name | teacher_age | teacher_gender |
+-------+---------------+-------------+----------------+---------+-----------+------+---------------+-------------+----------------+
| 2 | Shi Potian | 22 | M | 1 | 7 | NULL | NULL | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 | NULL | NULL | NULL | NULL |
| 6 | Shi Qing | 46 | M | 5 | NULL | NULL | NULL | NULL | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL | NULL | NULL | NULL | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL | NULL | NULL | NULL | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL | NULL | NULL | NULL | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL | NULL | NULL | NULL | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL | NULL | NULL | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL | NULL | NULL | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | NULL | 2 | Zhang Sanfeng | 94 | M |
+-------+---------------+-------------+----------------+---------+-----------+------+---------------+-------------+----------------+
23 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM (SELECT stuid,s.name AS student_name,s.age AS student_age,s.gender AS student_gender,classid,teacherid,tid,t.name AS teacher_name,t.age AS teacher_age,t.gender AS teacher_gender FROM students AS s LEFT OUTER JOIN teachers AS t ON s.TeacherID = t.TID UNION SELECT stuid,s.name AS student_name,s.age AS student_age,s.gender AS student_gender,classid,teacherid,tid,t.name AS teacher_name,t.age AS teacher_age,t.gender AS teacher_gender FROM students AS s RIGHT OUTER JOIN teachers AS t ON s.TeacherID = t.TID) AS f WHERE f.teacherid IS NULL OR f.tid IS NULL; #将间接实现完全外连接查询结果再次进行过滤,查询出完全外连接中过滤掉两表重复的数据。
6>.自连接查询
MariaDB [yinzhengjie]> CREATE TABLE emp(id int,name char(3),leaderid int);
Query OK, 0 rows affected (0.01 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> DESC emp;
+----------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | char(3) | YES | | NULL | |
| leaderid | int(11) | YES | | NULL | |
+----------+---------+------+-----+---------+-------+
rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> INSERT INTO emp VALUES(1,'张无忌',null),(2,'杨逍',1),(3,'范遥',2),(4,'殷天正',2),(5,'韦一笑',2);
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0 MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM emp;
+------+-----------+----------+
| id | name | leaderid |
+------+-----------+----------+
| 1 | 张无忌 | NULL |
| 2 | 杨逍 | 1 |
| 3 | 范遥 | 2 |
| 4 | 殷天正 | 2 |
| 5 | 韦一笑 | 2 |
+------+-----------+----------+
rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT a.name,b.name AS 领导 FROM emp AS a LEFT OUTER JOIN emp AS b ON a.leaderid = b.id; #同一张表查询数据，查询每个人及其领导的姓名。
+-----------+-----------+
| name | 领导 |
+-----------+-----------+
| 杨逍 | 张无忌 |
| 范遥 | 杨逍 |
| 殷天正 | 杨逍 |
| 韦一笑 | 杨逍 |
| 张无忌 | NULL |
+-----------+-----------+
5 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT a.name,b.name AS 领导 FROM emp AS a LEFT OUTER JOIN emp AS b ON a.leaderid = b.id; #同一张表查询数据，查询每个人及其领导的姓名。
三.SELECT语句(单表操作)执行流程
四.小试牛刀(本案例使用3张表，仍基于上面导入的"yinzhengjie_innodb.sql"数据进行演示)
MariaDB [yinzhengjie]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM students;
MariaDB [yinzhengjie]> SELECT * FROM courses;
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 3 | Jinshe Jianfa |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
+----------+----------------+
7 rows in set (0.01 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM courses;
MariaDB [yinzhengjie]> SELECT * FROM scores;
+----+-------+----------+-------+
| ID | StuID | CourseID | Score |
+----+-------+----------+-------+
| 1 | 1 | 2 | 77 |
| 2 | 1 | 6 | 93 |
| 3 | 2 | 2 | 47 |
| 4 | 2 | 5 | 97 |
| 5 | 3 | 2 | 88 |
| 6 | 3 | 6 | 75 |
| 7 | 4 | 5 | 71 |
| 8 | 4 | 2 | 89 |
| 9 | 5 | 1 | 39 |
| 10 | 5 | 7 | 63 |
| 11 | 6 | 1 | 96 |
| 12 | 7 | 1 | 86 |
| 13 | 7 | 7 | 83 |
| 14 | 8 | 4 | 57 |
| 15 | 8 | 3 | 93 |
+----+-------+----------+-------+
15 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM scores;
1>.查询学生姓名，考试科目以及对应的成绩
MariaDB [yinzhengjie]> SELECT a.name AS 姓名,c.course AS 课程,b.score AS 分数 FROM students AS a INNER JOIN scores AS b ON a.stuid = b.id INNER JOIN courses AS c ON b.courseid = c.courseid;
+---------------+----------------+--------+
| 姓名 | 课程 | 分数 |
+---------------+----------------+--------+
| Shi Zhongyu | Kuihua Baodian | 77 |
| Shi Potian | Weituo Zhang | 93 |
| Xie Yanke | Kuihua Baodian | 47 |
| Ding Dian | Daiyu Zanghua | 97 |
| Yu Yutong | Kuihua Baodian | 88 |
| Shi Qing | Weituo Zhang | 75 |
| Xi Ren | Daiyu Zanghua | 71 |
| Lin Daiyu | Kuihua Baodian | 89 |
| Ren Yingying | Hamo Gong | 39 |
| Yue Lingshan | Dagou Bangfa | 63 |
| Yuan Chengzhi | Hamo Gong | 96 |
| Wen Qingqing | Hamo Gong | 86 |
| Tian Boguang | Dagou Bangfa | 83 |
| Lu Wushuang | Taiji Quan | 57 |
| Duan Yu | Jinshe Jianfa | 93 |
+---------------+----------------+--------+
15 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT a.name AS 姓名,c.course AS 课程,b.score AS 分数 FROM students AS a INNER JOIN scores AS b ON a.stuid = b.id INNER JOIN courses AS c ON b.courseid = c.courseid;
2>.以ClassID分组，显示每班的同学的人数
3>.以Gender分组，显示其年龄之和
4>.以ClassID分组，显示其平均年龄大于25的班级
5>.以Gender分组，显示各组中年龄大于25的学员的年龄之和
6>.显示前5位同学的姓名、课程及成绩
7>.显示其成绩高于80的同学的名称及课程
8>.取每位同学各门课的平均成绩，显示成绩前三名的同学的姓名和平均成绩
9>.显示每门课程课程名称及学习了这门课的同学的个数
10>.显示其年龄大于平均年龄的同学的名字
11>.显示其学习的课程为第1、2，4或第7门课的同学的名字
12>.显示其成员数最少为3个的班级的同学中年龄大于同班同学平均年龄的同学
13>.统计各班级中年龄大于全校同学平均年龄的同学