MySQL 内置函数

CHAR_LENGTH(str)

返回值为字符串str 的长度，长度的单位为字符。一个多字节字符算作一个单字符。

+------------------------+
| CHAR_LENGTH('klvchen') |
+------------------------+
|                      7 |
+------------------------+
1 row in set (0.00 sec)

mysql> SELECT CHAR_LENGTH('你');
+--------------------+
| CHAR_LENGTH('你')  |
+--------------------+
|                  1 |
+--------------------+
1 row in set (0.00 sec)

CONCAT(str1,str2,...)

字符串拼接, 如有任何一个参数为NULL ，则返回值为 NULL。

mysql> SELECT CONCAT('kl','v','chen');
+-------------------------+
| CONCAT('kl','v','chen') |
+-------------------------+
| klvchen                 |
+-------------------------+
1 row in set (0.00 sec)

CONCAT_WS(separator,str1,str2,...)

字符串拼接（自定义连接符）CONCAT_WS()不会忽略任何空字符串。 (然而会忽略所有的 NULL）

mysql> SELECT CONCAT_WS('_','chen','wj');
+----------------------------+
| CONCAT_WS('_','chen','wj') |
+----------------------------+
| chen_wj                    |
+----------------------------+
1 row in set (0.01 sec)

mysql> SELECT CONCAT_WS('_','chen',' wj ', NULL, 'hehe');
+--------------------------------------------+
| CONCAT_WS('_','chen',' wj ', NULL, 'hehe') |
+--------------------------------------------+
| chen_ wj _hehe                             |
+--------------------------------------------+
1 row in set (0.00 sec)

CONV(N,from_base,to_base)

进制转换

mysql> SELECT CONV('a',16,2);
+----------------+
| CONV('a',16,2) |
+----------------+
| 1010           |
+----------------+
1 row in set (0.00 sec)

FORMAT(X,D)

将数字X 的格式写为'#,###,###.##',以四舍五入的方式保留小数点后 D 位， 并将结果以字符串的形式返回。若 D 为 0, 则返回结果不带有小数点，或不含小数部分。

mysql> SELECT FORMAT(1.23,4);
+----------------+
| FORMAT(1.23,4) |
+----------------+
| 1.2300         |
+----------------+
1 row in set (0.00 sec)

INSERT(str,pos,len,newstr)

str 原字符串; pos 要替换位置其实位置(从1开始); len：替换的长度；newstr：需要替换的符串
简洁概况为：先从 pos 处删除 len 长度的字符串，再由新的字符串代替

mysql> SELECT INSERT('klvchen', 1, 1, 'j');
+------------------------------+
| INSERT('klvchen', 1, 1, 'j') |
+------------------------------+
| jlvchen                      |
+------------------------------+
1 row in set (0.00 sec)

mysql> SELECT INSERT('klvchen', 1, 1, 'ja');
+-------------------------------+
| INSERT('klvchen', 1, 1, 'ja') |
+-------------------------------+
| jalvchen                      |
+-------------------------------+
1 row in set (0.00 sec)

mysql> SELECT INSERT('klvchen', 1, 2, 'a');
+------------------------------+
| INSERT('klvchen', 1, 2, 'a') |
+------------------------------+
| avchen                       |
+------------------------------+
1 row in set (0.00 sec)

INSTR(str,substr)

返回字符串 str 中子字符串的第一个出现位置。

mysql> SELECT INSTR('klvchen','l');
+----------------------+
| INSTR('klvchen','l') |
+----------------------+
|                    2 |
+----------------------+
1 row in set (0.00 sec)

LOCATE(substr,str,pos)

获取子序列索引位置

mysql> SELECT LOCATE('l','klvchen',1);
+-------------------------+
| LOCATE('l','klvchen',1) |
+-------------------------+
|                       2 |
+-------------------------+
1 row in set (0.01 sec)

LEFT(str,len)

返回字符串 str 从开始的 len 位置的子序列字符。

mysql> SELECT LEFT('klvchen',2);
+-------------------+
| LEFT('klvchen',2) |
+-------------------+
| kl                |
+-------------------+
1 row in set (0.00 sec)

RIGHT(str,len)

从字符串 str 开始，返回从后边开始 len个字符组成的子序列

mysql> SELECT RIGHT('klvchen', 2);
+---------------------+
| RIGHT('klvchen', 2) |
+---------------------+
| en                  |
+---------------------+
1 row in set (0.00 sec)

LOWER(str)

字符串转化为小写

mysql> SELECT LOWER('KLVCHEN');
+------------------+
| LOWER('KLVCHEN') |
+------------------+
| klvchen          |
+------------------+
1 row in set (0.00 sec)

UPPER(str)

字符串转化为大写

mysql> SELECT UPPER('klvchen');
+------------------+
| UPPER('klvchen') |
+------------------+
| KLVCHEN          |
+------------------+
1 row in set (0.00 sec)

LTRIM(str)

返回字符串 str ，左边的空格字符被删除。

mysql> SELECT LTRIM('  klvchen');
+--------------------+
| LTRIM('  klvchen') |
+--------------------+
| klvchen            |
+--------------------+
1 row in set (0.00 sec)

RTRIM(str)

返回字符串 str ，结尾空格字符被被删除。

mysql> SELECT RTRIM('klvchen    ');
+----------------------+
| RTRIM('klvchen    ') |
+----------------------+
| klvchen              |
+----------------------+
1 row in set (0.00 sec)

TRIM([{BOTH | LEADING | TRAILING} [remstr] FROM] str) TRIM(remstr FROM] str)

返回字符串 str ， 其中所有remstr 前缀和/或后缀都已被删除。若分类符BOTH、LEADIN或TRAILING中没有一个是给定的,则假设为BOTH 。 remstr 为可选项，在未指定情况下，可删除空格。

mysql> SELECT TRIM('   kl   ');
+------------------+
| TRIM('   kl   ') |
+------------------+
| kl               |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT TRIM(LEADING 'x' FROM  'xxklxx');
+----------------------------------+
| TRIM(LEADING 'x' FROM  'xxklxx') |
+----------------------------------+
| klxx                             |
+----------------------------------+
1 row in set (0.00 sec)

mysql> SELECT TRIM(BOTH 'x' FROM  'xxklxx');
+-------------------------------+
| TRIM(BOTH 'x' FROM  'xxklxx') |
+-------------------------------+
| kl                            |
+-------------------------------+
1 row in set (0.00 sec)

mysql> SELECT TRIM(trailing 'x' FROM  'xxklxx');
+-----------------------------------+
| TRIM(trailing 'x' FROM  'xxklxx') |
+-----------------------------------+
| xxkl                              |
+-----------------------------------+
1 row in set (0.00 sec)

REPEAT(str,count)

返回一个由重复的字符串 str 组成的字符串，字符串str重复的次数等于count的值 。
若 count <= 0,则返回一个空字符串；若str 或 count 为 NULL，则返回 NULL 。

mysql> SELECT REPEAT('ha',3);
+----------------+
| REPEAT('ha',3) |
+----------------+
| hahaha         |
+----------------+
1 row in set (0.00 sec)

REPLACE(str,from_str,to_str)

返回字符串str 以及所有被字符串to_str替代的字符串from_str 。

mysql> SELECT REPLACE('klvchel', 'l', 'i');
+------------------------------+
| REPLACE('klvchel', 'l', 'i') |
+------------------------------+
| kivchei                      |
+------------------------------+
1 row in set (0.00 sec)

REVERSE(str)

返回字符串 str ，顺序和字符顺序相反。

mysql> SELECT REVERSE('klvchen');
+--------------------+
| REVERSE('klvchen') |
+--------------------+
| nehcvlk            |
+--------------------+
1 row in set (0.00 sec)

SUBSTRING(str,pos,len)

获取字符串子序列

mysql> SELECT SUBSTRING('klvchen', 1, 3);
+----------------------------+
| SUBSTRING('klvchen', 1, 3) |
+----------------------------+
| klv                        |
+----------------------------+
1 row in set (0.00 sec)

mysql> SELECT SUBSTRING('klvchen', 3);
+-------------------------+
| SUBSTRING('klvchen', 3) |
+-------------------------+
| vchen                   |
+-------------------------+
1 row in set (0.00 sec)

mysql> SELECT SUBSTRING('klvchen' FROM 3);
+-----------------------------+
| SUBSTRING('klvchen' FROM 3) |
+-----------------------------+
| vchen                       |
+-----------------------------+
1 row in set (0.00 sec)

mysql> SELECT SUBSTRING('klvchen', -3);
+--------------------------+
| SUBSTRING('klvchen', -3) |
+--------------------------+
| hen                      |
+--------------------------+
1 row in set (0.00 sec)

mysql> SELECT SUBSTRING('klvchen', -3, 2);
+-----------------------------+
| SUBSTRING('klvchen', -3, 2) |
+-----------------------------+
| he                          |
+-----------------------------+
1 row in set (0.00 sec)

mysql> SELECT SUBSTRING('klvchen' FROM -3 FOR 2);
+------------------------------------+
| SUBSTRING('klvchen' FROM -3 FOR 2) |
+------------------------------------+
| he                                 |
+------------------------------------+
1 row in set (0.01 sec)