本文转载自:http://www.cnblogs.com/geaozhang/
是最常用的的一种函数,在一个具体应用中通常会综合几个甚至几类函数来实现相应的应用:
1、LOWER(column|str):将字符串参数值转换为全小写字母后返回
mysql> select lower('SQL Course');
+---------------------+
| lower('SQL Course') |
+---------------------+
| sql course |
+---------------------+
2、UPPER(column|str):将字符串参数值转换为全大写字母后返回
mysql> select upper('Use MYsql');
+--------------------+
| upper('Use MYsql') |
+--------------------+
| USE MYSQL |
+--------------------+
3、CONCAT(column|str1, column|str2,...):将多个字符串参数首尾相连后返回
mysql> select concat('My','S','QL');
+-----------------------+
| concat('My','S','QL') |
+-----------------------+
| MySQL |
+-----------------------+
如果有任何参数为null,则函数返回null
mysql> select concat('My',null,'QL');
+------------------------+
| concat('My',null,'QL') |
+------------------------+
| NULL |
+------------------------+
如果参数是数字,则自动转换为字符串
mysql> select concat(14.3,'mysql');
+----------------------+
| concat(14.3,'mysql') |
+----------------------+
| 14.3mysql |
+----------------------+
4、CONCAT_WS(separator,str1,str2,...):将多个字符串参数以给定的分隔符separator首尾相连后返回
mysql> select concat_ws(';','First name','Second name','Last name');
+-------------------------------------------------------+
| concat_ws(';','First name','Second name','Last name') |
+-------------------------------------------------------+
| First name;Second name;Last name |
+-------------------------------------------------------+
!!也就是函数圆括号里的第一个项目用来指定分隔符
注意:如果有任何参数为null,则函数不返回null,而是直接忽略它
mysql> select concat_ws(',','id',null,'name');
+---------------------------------+
| concat_ws(',','id',null,'name') |
+---------------------------------+
| id,name |
+---------------------------------+
打开和关闭管道符号“|”的连接功能
PIPES_AS_CONCAT:将“||”视为字符串的连接操作符而非或运算符
|| 管道连接符:
基本格式:
mysql> select 列名1 || 列名2 || 列名3 from 表名;
在mysql中,进行上式连接查询之后,会将查询结果集在一列中显示(字符串连接),列名是‘列名1 || 列名2 || 列名3’;
mysql> select s_no || s_name || s_age
-> from student;
+-------------------------+
| s_no || s_name || s_age |
+-------------------------+
| 1001张三23 |
| 1002李四19 |
| 1003马五20 |
| 1004甲六17 |
| 1005乙七22 |
+-------------------------+
注意:
①如果不显示结果,是因为sql_mode参数中没有PIPES_AS_CONCAT,只要给sql_mode参数加入PIPES_AS_CONCAT,就可以实现像CONCAT一样的功能;
②如果不给sql_mode参数加入PIPES_AS_CONCAT的话,|| 默认是or的意思,查询结果是一列显示是1。
5、SUBSTR(str,pos[,len]):从源字符串str中的指定位置pos开始取一个字串并返回
注意:
①len指定子串的长度,如果省略则一直取到字符串的末尾;len为负值表示从源字符串的尾部开始取起。
②函数SUBSTR()是函数SUBSTRING()的同义词。
mysql> select substring('hello world',5);
+----------------------------+
| substring('hello world',5) |
+----------------------------+
| o world |
+----------------------------+ mysql> select substr('hello world',5,3);
+---------------------------+
| substr('hello world',5,3) |
+---------------------------+
| o w |
+---------------------------+ mysql> select substr('hello world',-5);
+--------------------------+
| substr('hello world',-5) |
+--------------------------+
| world |
+--------------------------+
6、LENGTH(str):返回字符串的存储长度
mysql> select length('text'),length('你好');
+----------------+------------------+
| length('text') | length('你好') |
+----------------+------------------+
| 4 | 6 |
+----------------+------------------+
注意:编码方式不同字符串的存储长度就不一样(‘你好’:utf8是6,gbk是4)
7、CHAR_LENGTH(str):返回字符串中的字符个数
mysql> select char_length('text'),char_length('你好');
+---------------------+-----------------------+
| char_length('text') | char_length('你好') |
+---------------------+-----------------------+
| 4 | 2 |
+---------------------+-----------------------+
8、INSTR(str, substr):从源字符串str中返回子串substr第一次出现的位置
mysql> select instr('foobarbar','bar');
+--------------------------+
| instr('foobarbar','bar') |
+--------------------------+
| 4 |
+--------------------------+
9、LPAD(str, len, padstr):在源字符串的左边填充给定的字符padstr到指定的长度len,返回填充后的字符串
mysql> select lpad('hi',5,'??');
+-------------------+
| lpad('hi',5,'??') |
+-------------------+
| ???hi |
+-------------------+
10、RPAD(str, len, padstr):在源字符串的右边填充给定的字符padstr到指定的长度len,返回填充后的字符串
mysql> select rpad('hi',6,'??');
+-------------------+
| rpad('hi',6,'??') |
+-------------------+
| hi???? |
+-------------------+
11、TRIM([{BOTH | LEADING | TRAILING} [remstr] FROM] str), TRIM([remstr FROM] str):
从源字符串str中去掉两端、前缀或后缀字符remstr并返回;
如果不指定remstr,则去掉str两端的空格;
不指定BOTH、LEADING、TRAILING ,则默认为 BOTH。
mysql> select trim(' bar ');
+-----------------+
| trim(' bar ') |
+-----------------+
| bar |
+-----------------+ mysql> select trim(leading 'x' from 'xxxbarxxx');
+------------------------------------+
| trim(leading 'x' from 'xxxbarxxx') |
+------------------------------------+
| barxxx |
+------------------------------------+ mysql> select trim(both 'x' from 'xxxbarxxx');
+---------------------------------+
| trim(both 'x' from 'xxxbarxxx') |
+---------------------------------+
| bar |
+---------------------------------+ mysql> select trim(trailing 'xyz' from 'barxxyz');
+-------------------------------------+
| trim(trailing 'xyz' from 'barxxyz') |
+-------------------------------------+
| barx |
+-------------------------------------+
12、REPLACE(str, from_str, to_str):在源字符串str中查找所有的子串form_str(大小写敏感),找到后使用替代字符串to_str替换它。返回替换后的字符串
mysql> select replace('www.mysql.com','w','Ww');
+-----------------------------------+
| replace('www.mysql.com','w','Ww') |
+-----------------------------------+
| WwWwWw.mysql.com |
+-----------------------------------+
13、LTRIM(str),RTRIM(str):去掉字符串的左边或右边的空格(左对齐、右对齐)
mysql> SELECT ltrim(' barbar ') rs1, rtrim(' barbar ') rs2;
+-----------+-----------+
| rs1 | rs2 |
+-----------+-----------+
| barbar | barbar |
+-----------+-----------+
14、REPEAT(str, count):将字符串str重复count次后返回
mysql> select repeat('MySQL',3);
+-------------------+
| repeat('MySQL',3) |
+-------------------+
| MySQLMySQLMySQL |
+-------------------+
15、REVERSE(str):将字符串str反转后返回
mysql> select reverse('abcdef');
+-------------------+
| reverse('abcdef') |
+-------------------+
| fedcba |
+-------------------+
16、CHAR(N,... [USING charset_name]):将每个参数N解释为整数(字符的编码),并返回每个整数对应的字符所构成的字符串(NULL值被忽略)。
mysql> select char(77,121,83,81,'76'),char(77,77.3,'77.3');
+-------------------------+----------------------+
| char(77,121,83,81,'76') | char(77,77.3,'77.3') |
+-------------------------+----------------------+
| MySQL | MMM |
+-------------------------+----------------------+
默认情况下,函数返回二进制字符串,若想返回针对特定字符集的字符串,使用using选项
mysql> SELECT charset(char(0x65)), charset(char(0x65 USING utf8));
+---------------------+--------------------------------+
| charset(char(0x65)) | charset(char(0x65 USING utf8)) |
+---------------------+--------------------------------+
| binary | utf8 |
+---------------------+--------------------------------+
17、FORMAT(X,D[,locale]):以格式‘#,###,###.##’格式化数字X
D指定小数位数
locale指定国家语言(默认的locale为en_US)
mysql> SELECT format(12332.123456, 4),format(12332.2,0);
+-------------------------+-------------------+
| format(12332.123456, 4) | format(12332.2,0) |
+-------------------------+-------------------+
| 12,332.1235 | 12,332 |
+-------------------------+-------------------+ mysql> SELECT format(12332.2,2,'de_DE');
+---------------------------+
| format(12332.2,2,'de_DE') |
+---------------------------+
| 12.332,20 |
+---------------------------+
18、SPACE(N):返回由N个空格构成的字符串
mysql> select space(3);
+----------+
| space(3) |
+----------+
| |
+----------+
19、LEFT(str, len):返回最左边的len长度的子串
mysql> select left('chinaitsoft',5);
+-----------------------+
| left('chinaitsoft',5) |
+-----------------------+
| china |
+-----------------------+
20、RIGHT(str, len):返回最右边的len长度的子串
mysql> select right('chinaitsoft',5);
+------------------------+
| right('chinaitsoft',5) |
+------------------------+
| tsoft |
+------------------------+
21、STRCMP(expr1,expr2):如果两个字符串是一样的则返回0;如果第一个小于第二个则返回-1;否则返回1
mysql> select strcmp('text','text');
+-----------------------+
| strcmp('text','text') |
+-----------------------+
| 0 |
+-----------------------+ mysql> SELECT strcmp('text', 'text2'),strcmp('text2', 'text');
+-------------------------+-------------------------+
| strcmp('text', 'text2') | strcmp('text2', 'text') |
+-------------------------+-------------------------+
| -1 | 1 |
+-------------------------+-------------------------+