【MySQL】内置函数——字符串函数

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• 查看字符串的字符集——charset()

mysql> select ename from emp;
+--------+
| ename  |
+--------+
| SMITH  |
| ALLEN  |
| WARD   |
| JONES  |
| MARTIN |
| BLAKE  |
| CLARK  |
| SCOTT  |
| KING   |
| TURNER |
| ADAMS  |
| JAMES  |
| FORD   |
| MILLER |
+--------+

mysql> select charset(ename) from emp;
+----------------+
| charset(ename) |
+----------------+
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
+----------------+
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• 拼接字符串——concat()

mysql> select concat(deptno,'号部门的',ename,'的薪资是',sal) as 薪资 from emp limit 2;
+----------------------------------------+
| 薪资                                   |
+----------------------------------------+
| 20号部门的SMITH的薪资是800.00          |
| 30号部门的ALLEN的薪资是1600.00         |
+----------------------------------------+
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• string的长度——length()

mysql> select ename,length(ename) from emp limit 3;
+-------+---------------+
| ename | length(ename) |
+-------+---------------+
| SMITH |             5 |
| ALLEN |             5 |
| WARD  |             4 |
+-------+---------------+
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注意：length函数返回字符串长度，以字节为单位，如果是多字节字符则计算多个字节数；如果是单字节字符则算作一个字节。比如：字母，数字算作一个字节，中文算作多个字节（gbk中2字节，utf8中3字节）

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• 字符串指定替换——replace()

mysql> select ename from emp;
+--------+
| ename  |
+--------+
| SMITH  |
| ALLEN  |
| WARD   |
| JONES  |
| MARTIN |
| BLAKE  |
| CLARK  |
| SCOTT  |
| KING   |
| TURNER |
| ADAMS  |
| JAMES  |
| FORD   |
| MILLER |
+--------+
14 rows in set (0.00 sec)

mysql> select replace(ename,'S','上海') from emp;
+-----------------------------+
| replace(ename,'S','上海')   |
+-----------------------------+
| 上海MITH                    |
| ALLEN                       |
| WARD                        |
| JONE上海                    |
| MARTIN                      |
| BLAKE                       |
| CLARK                       |
| 上海COTT                    |
| KING                        |
| TURNER                      |
| ADAM上海                    |
| JAME上海                    |
| FORD                        |
| MILLER                      |
+-----------------------------+
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replace的替换会形成一张新表，原表数据并不会改变

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• 截取子字符串——substring()
• 从左往右截取——left()
• 从右往左截取——right()

//从ename属性数据的第二个字符开始，截取两个字符
mysql> select substring(ename,2,2) from emp limit 4;
+----------------------+
| substring(ename,2,2) |
+----------------------+
| MI                   |
| LL                   |
| AR                   |
| ON                   |
+----------------------+

mysql> select left(ename,2) from emp limit 4;
+---------------+
| left(ename,2) |
+---------------+
| SM            |
| AL            |
| WA            |
| JO            |
+---------------+

mysql> select right(ename,2) from emp limit 4;
+----------------+
| right(ename,2) |
+----------------+
| TH             |
| EN             |
| RD             |
| ES             |
+----------------+
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转换成大写——ucase()
转换成小写——lcase()

mysql> select ucase('hello');
+----------------+
| ucase('hello') |
+----------------+
| HELLO          |
+----------------+
1 row in set (0.00 sec)

mysql> select lcase(ename) from emp limit 4;
+--------------+
| lcase(ename) |
+--------------+
| smith        |
| allen        |
| ward         |
| jones        |
+--------------+
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查询字符位置——instr()
注意：下标从1开始，没有则返回0

mysql> select instr('hello','ll');
+---------------------+
| instr('hello','ll') |
+---------------------+
|                   3 |
+---------------------+

mysql> select instr('hello','le');
+---------------------+
| instr('hello','le') |
+---------------------+
|                   0 |
+---------------------+
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逐字节比较两字符串大小——strcmp()
大于返回1，小于返回-1

mysql> select strcmp('hello','hl');
+----------------------+
| strcmp('hello','hl') |
+----------------------+
|                   -1 |
+----------------------+

mysql> select strcmp('hello','ha');
+----------------------+
| strcmp('hello','ha') |
+----------------------+
|                    1 |
+----------------------+
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去除左空格——ltrim()
去除右空格——rtrim()
去除左右空格——trim()

//去除左空格
mysql> select ('     你好   ') 空格 ;
+----------------+
| 空格           |
+----------------+
|      你好      |
+----------------+

mysql> select ltrim('     你好   ') 空格 ;
+-----------+
| 空格      |
+-----------+
| 你好      |
+-----------+

//去除右空格
mysql> select rtrim('     你好   ') 空格 ;
+-------------+
| 空格        |
+-------------+
|      你好   |
+-------------+

//去除左右空格
mysql> select trim('     你好   ') 空格 ;
+--------+
| 空格   |
+--------+
| 你好   |
+--------+
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