MySQL单表查询

时间:2022-01-24 03:15:00

MySQL单表查询

一、单表查询的语法及关键字执行的优先级

1.1 单表查询语法

SELECT DISTINCT 字段1,字段2... FROM 表名
                              WHERE 条件
                              GROUP BY field
                              HAVING 筛选
                              ORDER BY field
                              LIMIT 限制条数

1.2 关键字执行的优先级

  • from:找到表
  • where:拿着where指定的约束条件,去文件/表中取出一条条记录
  • group by:将取出的一条条记录进行分组group by,如果没有group by,则整体作为一组
  • select:执行select
  • distinct:去重
  • having:将分组的结果进行having过滤
  • order by:将结果按条件排序:order by
  • limit:限制结果的显示条数

二、简单查询

2.1 建表和数据准备

company.employee
    员工id      id                  int             
    姓名        emp_name            varchar
    性别        sex                 enum
    年龄        age                 int
    入职日期     hire_date           date
    岗位        post                varchar
    职位描述     post_comment        varchar
    薪水        salary              double
    办公室       office              int
    部门编号     depart_id           int



# 创建表
create table employee(
id int not null unique auto_increment,
emp_name varchar(20) not null,
sex enum('male','female') not null default 'male',  #  大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int,  #  一个部门一个屋子
depart_id int
);


# 查看表结构
mysql> desc employee;
 -------------- ----------------------- ------ ----- --------- ---------------- 
| Field        | Type                  | Null | Key | Default | Extra          |
 -------------- ----------------------- ------ ----- --------- ---------------- 
| id           | int(11)               | NO   | PRI | NULL    | auto_increment |
| emp_name     | varchar(20)           | NO   |     | NULL    |                |
| sex          | enum('male','female') | NO   |     | male    |                |
| age          | int(3) unsigned       | NO   |     | 28      |                |
| hire_date    | date                  | NO   |     | NULL    |                |
| post         | varchar(50)           | YES  |     | NULL    |                |
| post_comment | varchar(100)          | YES  |     | NULL    |                |
| salary       | double(15,2)          | YES  |     | NULL    |                |
| office       | int(11)               | YES  |     | NULL    |                |
| depart_id    | int(11)               | YES  |     | NULL    |                |
 -------------- ----------------------- ------ ----- --------- ---------------- 

# 插入记录
# 三个部门:教学,销售,运营
insert into employee(emp_name,sex,age,hire_date,post,salary,office,depart_id) 
values
('nick','male',18,'20170301','老男孩驻上海虹桥最帅',7300.33,401,1), # 以下是教学部
('jason','male',78,'20150302','teacher',1000000.31,401,1),
('sean','male',81,'20130305','teacher',8300,401,1),
('tank','male',73,'20140701','teacher',3500,401,1),
('oscar','male',28,'20121101','teacher',2100,401,1),
('mac','female',18,'20110211','teacher',9000,401,1),
('rocky','male',18,'19000301','teacher',30000,401,1),
('成龙','male',48,'20101111','teacher',10000,401,1),

('歪歪','female',48,'20150311','sale',3000.13,402,2),   # 以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),

('张野','male',28,'20160311','operation',10000.13,403,3),  # 以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;

#  ps:如果在windows系统中,插入中文字符,select的结果为空白,可以将所有字符编码统一设置成gbk
# 简单查询
    SELECT id,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
    FROM employee;

    SELECT * FROM employee;

    SELECT emp_name,salary FROM employee;

# 避免重复DISTINCT
    SELECT DISTINCT post FROM employee;    

# 通过四则运算查询
    SELECT emp_name, salary*12 FROM employee;
    SELECT emp_name, salary*12 AS Annual_salary FROM employee;
    SELECT emp_name, salary*12 Annual_salary FROM employee;

# 定义显示格式
   CONCAT() 函数用于连接字符串
   SELECT CONCAT('姓名: ',emp_name,'  年薪: ', salary*12)  AS Annual_salary 
   FROM employee;
   
   CONCAT_WS() 第一个参数为分隔符
   SELECT CONCAT_WS(':',emp_name,salary*12)  AS Annual_salary 
   FROM employee;

   结合CASE语句:
   SELECT
       (
           CASE
           WHEN emp_name = 'mac' THEN
               emp_name
           WHEN emp_name = 'jason' THEN
               CONCAT(emp_name,'_BIGSB')
           ELSE
               concat(emp_name, 'SB')
           END
       ) as new_name
   FROM
       employee;

2.2 练习

  • 查出所有员工的名字,薪资,格式为<名字:nick> <薪资:3000>
  • 查出所有的岗位(去掉重复)
  • 查出所有员工名字,以及他们的年薪,年薪的字段名为annual_year
select concat('<名字:',emp_name,'>    ','<薪资:',salary,'>') from employee;
select distinct depart_id from employee;
select emp_name,salary*12 annual_salary from employee;

三、约束条件(where)

where子句中可以使用:

  • 比较运算符:> < >= <= <> !=
  • between 80 and 100 值在80到100之间
  • in(80,90,100) 值是80或90或100
  • like ‘n%‘

  • 通配符可以是%或_,

  • %表示任意多字符
  • _表示一个字符

  • 逻辑运算符:在多个条件直接可以使用逻辑运算符 and or not

1. 单条件查询
    SELECT emp_name FROM employee
        WHERE post='sale';
        
2. 多条件查询
    SELECT emp_name,salary FROM employee
        WHERE post='teacher' AND salary>10000;

3. 关键字BETWEEN AND
    SELECT emp_name,salary FROM employee 
        WHERE salary BETWEEN 10000 AND 20000;

    SELECT emp_name,salary FROM employee 
        WHERE salary NOT BETWEEN 10000 AND 20000;
    
4. 关键字IS NULL(判断某个字段是否为NULL不能用等号,需要用IS)
    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment IS NULL;

    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment IS NOT NULL;
        
    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment=''; 注意''是空字符串,不是null
    ps:
        执行
        update employee set post_comment='' where id=2;
        再用上条查看,就会有结果了

5. 关键字IN集合查询
    SELECT emp_name,salary FROM employee 
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT emp_name,salary FROM employee 
        WHERE salary IN (3000,3500,4000,9000) ;

    SELECT emp_name,salary FROM employee 
        WHERE salary NOT IN (3000,3500,4000,9000) ;

6. 关键字LIKE模糊查询
    通配符’%’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'ni%';

    通配符’_’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'ja__';

3.1 练习

  • 查看岗位是teacher的员工姓名、年龄
  • 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
  • 查看岗位是teacher且薪资在9000-1000范围内的员工姓名、年龄、薪资
  • 查看岗位描述不为NULL的员工信息
  • 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资
  • 查看岗位是teacher且薪资不是10000或9000或30000的员工姓名、年龄、薪资
  • 查看岗位是teacher且名字是jin开头的员工姓名、年薪
select emp_name,age from employee where post = 'teacher';
select emp_name,age from employee where post='teacher' and age > 30; 
select emp_name,age,salary from employee where post='teacher' and salary between 9000 and 10000;
select * from employee where post_comment is not null;
select emp_name,age,salary from employee where post='teacher' and salary in (10000,9000,30000);
select emp_name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000);
select emp_name,salary*12 from employee where post='teacher' and emp_name like 'mac%';

四、分组(group by)

单独使用GROUP BY关键字分组
    SELECT post FROM employee GROUP BY post;
    注意:我们按照post字段分组,那么select查询的字段只能是post,
    想要获取组内的其他相关信息,需要借助函数

GROUP BY关键字和GROUP_CONCAT()函数一起使用
    SELECT post,GROUP_CONCAT(emp_name) FROM employee GROUP BY post;  # 按照岗位分组,并查看组内成员名
    SELECT post,GROUP_CONCAT(emp_name) as emp_members FROM employee GROUP BY post;

GROUP BY与聚合函数一起使用
    select post,count(id) as count from employee group by post;  #  按照岗位分组,并查看每个组有多少人

注意:如果我们用unique的字段作为分组的依据,则每一条记录自成一组,这种分组没有意义;多条记录之间的某个字段值相同,该字段通常用来作为分组的依据。

五、聚合函数

强调:聚合函数聚合的是组的内容,若是没有分组,则默认一组。
示例:

    SELECT COUNT(*) FROM employee;
    SELECT COUNT(*) FROM employee WHERE depart_id=1;
    SELECT MAX(salary) FROM employee;
    SELECT MIN(salary) FROM employee;
    SELECT AVG(salary) FROM employee;
    SELECT SUM(salary) FROM employee;
    SELECT SUM(salary) FROM employee WHERE depart_id=3;

5.1 练习

  • 查询岗位名以及岗位包含的所有员工名字
  • 查询岗位名以及各岗位内包含的员工个数
  • 查询公司内男员工和女员工的个数
  • 查询岗位名以及各岗位的平均薪资
  • 查询岗位名以及各岗位的最高薪资
  • 查询岗位名以及各岗位的最低薪资
  • 查询男员工与男员工的平均薪资,女员工与女员工的平均薪资
题目1
mysql> select post,group_concat(emp_name) from employee group by post;
 ---------------------------- ------------------------------------- 
| post                       | count(id)                           |
|operation                   | 张野,程咬金,程咬银,程咬铜,程咬铁      |  
| sale                       | 歪歪,丫丫,丁丁,星星,格格              |
| teacher                    | jason,sean,tank,oscar,mac,rocky,成龙 |
| 老男孩驻上海虹桥最帅        | nick                                 |
 ---------------------------- ------------------------------------- 


题目2
mysql> select post,count(id) from employee group by post;
 ----------------------------------------- ----------- 
| post                                    | count(id) |
 ----------------------------------------- ----------- 
| operation                               |         5 |
| sale                                    |         5 |
| teacher                                 |         7 |
| 老男孩驻上海虹桥最帅              |         1 |
 ----------------------------------------- ----------- 


题目3
mysql> select sex,count(id) from employee group by sex;
 -------- ----------- 
| sex    | count(id) |
 -------- ----------- 
| male   |        10 |
| female |         8 |
 -------- ----------- 

题目4
mysql> select post,avg(salary) from employee group by post;
 ----------------------------------------- --------------- 
| post                                    | avg(salary)   |
 ----------------------------------------- --------------- 
| operation                               |  16800.026000 |
| sale                                    |   2600.294000 |
| teacher                                 | 151842.901429 |
| 老男孩驻上海虹桥最帅              |   7300.330000 |
 ----------------------------------------- --------------- 

题目5
mysql> select post,max(salary) from employee group by post;
 ----------------------------------------- ------------- 
| post                                    | max(salary) |
 ----------------------------------------- ------------- 
| operation                               |    20000.00 |
| sale                                    |     4000.33 |
| teacher                                 |  1000000.31 |
| 老男孩驻上海虹桥最帅              |     7300.33 |
 ----------------------------------------- ------------- 

题目6
mysql> select post,min(salary) from employee group by post;
 ----------------------------------------- ------------- 
| post                                    | min(salary) |
 ----------------------------------------- ------------- 
| operation                               |    10000.13 |
| sale                                    |     1000.37 |
| teacher                                 |     2100.00 |
| 老男孩驻上海虹桥最帅              |     7300.33 |
 ----------------------------------------- ------------- 

题目7
mysql> select sex,avg(salary) from employee group by sex;
 -------- --------------- 
| sex    | avg(salary)   |
 -------- --------------- 
| male   | 110920.077000 |
| female |   7250.183750 |
 -------- --------------- 

六、过滤(having)

6.1 where和having的区别

执行优先级从高到低:where > group by > having

  • Where 发生在分组group by之前,因而Where中可以有任意字段,但是绝对不能使用聚合函数。
  • Having发生在分组group by之后,因而Having中可以使用分组的字段,无法直接取到其他字段,可以使用聚合函数

6.1.1 验证

mysql> select @@sql_mode;
 -------------------- 
| @@sql_mode         |
 -------------------- 
| ONLY_FULL_GROUP_BY |
 -------------------- 
row in set (0.00 sec)

mysql> select * from emp where salary > 100000;
 ---- ------ ------ ----- ------------ --------- -------------- ------------ -------- ----------- 
| id | emp_name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
 ---- ------ ------ ----- ------------ --------- -------------- ------------ -------- ----------- 
|  2 | jason | male |  78 | 2015-03-02 | teacher | NULL         | 1000000.31 |    401 |         1 |
 ---- ------ ------ ----- ------------ --------- -------------- ------------ -------- ----------- 
row in set (0.00 sec)


mysql> select post,group_concat(emp_name) from emp group by post having salary > 10000;#错误,分组后无法直接取到salary字段
ERROR 1054 (42S22): Unknown column 'salary' in 'having clause'
mysql> select post,group_concat(emp_name) from emp group by post having avg(salary) > 10000;
 ----------- ------------------------------------------------------- 
| post | group_concat(emp_name) |
 ----------- ------------------------------------------------------- 
| operation | 程咬铁,程咬铜,程咬银,程咬金,张野 |
| teacher | 成龙,rocky,mac,oscar,tank,sean,jason |
 ----------- ------------------------------------------------------- 
rows in set (0.00 sec)

6.2 练习

  • 查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数
  • 查询各岗位平均薪资大于10000的岗位名、平均工资
  • 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
题目1
mysql> select post,group_concat(emp_name),count(id) from employee group by post having count(id) < 2;
 ---------------------- ------------------------ ----------- 
| post                 | group_concat(emp_name) | count(id) |
 ---------------------- ------------------------ ----------- 
| 老男孩驻上海虹桥最帅  | nick                   |         1 |
 ----------------------- ----------------------- ----------- 

题目2
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;
 ----------- --------------- 
| post      | avg(salary)   |
 ----------- --------------- 
| operation |  16800.026000 |
| teacher   | 151842.901429 |
 ----------- --------------- 

题目3
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000;
 ----------- -------------- 
| post      | avg(salary)  |
 ----------- -------------- 
| operation | 16800.026000 |
 ----------- -------------- 

七、查询排序(order by)

按单列排序
    SELECT * FROM employee ORDER BY salary;
    SELECT * FROM employee ORDER BY salary ASC;
    SELECT * FROM employee ORDER BY salary DESC;

按多列排序:先按照age排序,如果年纪相同,则按照薪资排序
    SELECT * from employee
        ORDER BY age,
        salary DESC;

7.1 练习

  • 查询所有员工信息,先按照age升序排序,如果age相同则按照hire_date降序排序
  • 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列
  • 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列
题目1
mysql> select * from employee ORDER BY age asc,hire_date desc;

题目2
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc;
 ----------- --------------- 
| post      | avg(salary)   |
 ----------- --------------- 
| operation |  16800.026000 |
| teacher   | 151842.901429 |
 ----------- --------------- 

题目3
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc;
 ----------- --------------- 
| post      | avg(salary)   |
 ----------- --------------- 
| teacher   | 151842.901429 |
| operation |  16800.026000 |
 ----------- --------------- 

八、限制查询的记录数(limit)

示例:

SELECT * FROM employee ORDER BY salary DESC 
    LIMIT 3;                    #默认初始位置为0 

SELECT * FROM employee ORDER BY salary DESC
    LIMIT 0,5; #从第0开始,即先出第一条,然后包含这一条在内往后查5条

SELECT * FROM employee ORDER BY salary DESC
    LIMIT 5,5; #从第5开始,即先出第6条,然后包含这一条在内往后查5条

8.1 练习

  • 分页显示,每页5条
mysql> select * from  employee limit 0,5;
 ---- ----------- ------ ----- ------------ ----------------------------------------- -------------- ------------ -------- ----------- 
| id | emp_name      | sex  | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
 ---- ----------- ------ ----- ------------ ----------------------------------------- -------------- ------------ -------- ----------- 
|  1 | nick      | male |  18 | 2017-03-01 | 老男孩驻上海虹桥最帅              | NULL         |    7300.33 |    401 |         1 |
|  2 | jason      | male |  78 | 2015-03-02 | teacher                                 |              | 1000000.31 |    401 |         1 |
|  3 | sean   | male |  81 | 2013-03-05 | teacher                                 | NULL         |    8300.00 |    401 |         1 |
|  4 | tank   | male |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    401 |         1 |
|  5 | oscar | male |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 |
 ---- ----------- ------ ----- ------------ ----------------------------------------- -------------- ------------ -------- ----------- 
rows in set (0.00 sec)


mysql> select * from  employee limit 5,5;
 ---- ------------ -------- ----- ------------ --------- -------------- ---------- -------- ----------- 
| id | emp_name       | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |
 ---- ------------ -------- ----- ------------ --------- -------------- ---------- -------- ----------- 
|  6 | mac | female |  18 | 2011-02-11 | teacher | NULL         |  9000.00 |    401 |         1 |
|  7 | rocky     | male   |  18 | 1900-03-01 | teacher | NULL         | 30000.00 |    401 |         1 |
|  8 | 成龙       | male   |  48 | 2010-11-11 | teacher | NULL         | 10000.00 |    401 |         1 |
|  9 | 歪歪       | female |  48 | 2015-03-11 | sale    | NULL         |  3000.13 |    402 |         2 |
| 10 | 丫丫       | female |  38 | 2010-11-01 | sale    | NULL         |  2000.35 |    402 |         2 |
 ---- ------------ -------- ----- ------------ --------- -------------- ---------- -------- ----------- 
rows in set (0.00 sec)


mysql> select * from  employee limit 10,5;
 ---- ----------- -------- ----- ------------ ----------- -------------- ---------- -------- ----------- 
| id | emp_name      | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
 ---- ----------- -------- ----- ------------ ----------- -------------- ---------- -------- ----------- 
| 11 | 丁丁      | female |  18 | 2011-03-12 | sale      | NULL         |  1000.37 |    402 |         2 |
| 12 | 星星      | female |  18 | 2016-05-13 | sale      | NULL         |  3000.29 |    402 |         2 |
| 13 | 格格      | female |  28 | 2017-01-27 | sale      | NULL         |  4000.33 |    402 |         2 |
| 14 | 张野      | male   |  28 | 2016-03-11 | operation | NULL         | 10000.13 |    403 |         3 |
| 15 | 程咬金    | male   |  18 | 1997-03-12 | operation | NULL         | 20000.00 |    403 |         3 |
 ---- ----------- -------- ----- ------------ ----------- -------------- ---------- -------- ----------- 
rows in set (0.00 sec)

九、使用正则表达式查询

SELECT * FROM employee WHERE emp_name REGEXP '^jas';

SELECT * FROM employee WHERE emp_name REGEXP 'on$';

SELECT * FROM employee WHERE emp_name REGEXP 'm{2}';


小结:对字符串匹配的方式
WHERE emp_name = 'nick';
WHERE emp_name LIKE 'sea%';
WHERE emp_name REGEXP 'on$';

9.1 练习

  • 查看所有员工中名字是mac开头,n或者g结果的员工信息
select * from employee where emp_name regexp '^mac.*[gn]$';