mysql---字符串函数

1,concat

mysql> select concat('aaa', 'bbb', 'cccc'), concat('aaa', null); +------------------------------+---------------------+
| concat('aaa', 'bbb', 'cccc') | concat('aaa', null) | +------------------------------+---------------------+
| aaabbbcccc | NULL | +------------------------------+---------------------+
1 row in set (0.01 sec)

含有null的拼接返回null，否则直接拼接
2,insert

mysql> select insert('beijing2008', 8, 4, 'me'); +-----------------------------------+
| insert('beijing2008', 8, 4, 'me') | +-----------------------------------+
| beijingme | +-----------------------------------+
1 row in set (0.00 sec)

将beijing2008字符串从第八位后的四位字符串替换成me；
3，lower upper

mysql> select lower('Aaaaa'); +----------------+
| lower('Aaaaa') | +----------------+
| aaaaa | +----------------+
1 row in set (0.00 sec)

mysql> select uper('Abplc');
ERROR 1305 (42000): FUNCTION user.uper does not exist
mysql> select upper('Abdasf'); +-----------------+
| upper('Abdasf') | +-----------------+
| ABDASF | +-----------------+
1 row in set (0.00 sec)

lower 全部转成小写，upper 全部转成大写

4,left right

mysql> select left('beijing2008', 4), left('beijing2008', null), right('beijing2008', 4); +------------------------+---------------------------+-------------------------+
| left('beijing2008', 4) | left('beijing2008', null) | right('beijing2008', 4) | +------------------------+---------------------------+-------------------------+
| beij | NULL | 2008 | +------------------------+---------------------------+-------------------------+
1 row in set (0.00 sec)

left 显示字符串左边指定长度字符串， right显示字符串右边指定长度字符串
5，lpad， rpad

mysql> select lpad('2008',30,'beijing'), rpad('beijing',20,'2008'); +--------------------------------+---------------------------+
| lpad('2008',30,'beijing') | rpad('beijing',20,'2008') | +--------------------------------+---------------------------+
| beijingbeijingbeijingbeiji2008 | beijing2008200820082 | +--------------------------------+---------------------------+
1 row in set (0.00 sec)

用beijing对2008左填充直到字符串长度为30，用2008对beijing右填充直到字符串长度为20；
6,ltrim,rtrim

mysql> select ltrim(' beijing | '), rtrim(' beijing | '); +----------------------+----------------------+
| ltrim(' beijing | ') | rtrim(' beijing | ') | +----------------------+----------------------+
| beijing | | beijing | | +----------------------+----------------------+
1 row in set (0.00 sec)

对beijing进行左侧去空格和右侧去空格
7,repeat

mysql> select repeat('beijing', 3); +-----------------------+
| repeat('beijing', 3) | +-----------------------+
| beijingbeijingbeijing | +-----------------------+
1 row in set (0.00 sec)

对beijing进行三次重复
8,replace

mysql> select replace('beijing_2010_2010', '2010', '2008'); +----------------------------------------------+
| replace('beijing_2010_2010', '2010', '2008') | +----------------------------------------------+
| beijing_2008_2008 | +----------------------------------------------+
1 row in set (0.01 sec)

将所有的2010全部替换成2008
9,strcmp

mysql> select strcmp('a', 'b'), strcmp('b', 'a'); +------------------+------------------+
| strcmp('a', 'b') | strcmp('b', 'a') | +------------------+------------------+
| -1 | 1 | +------------------+------------------+
1 row in set (0.00 sec)

用来比较字符串的ASSCII码大小，前者小于后者返回-1，相等返回0， 大于后者返回1
10,trim

mysql> select trim(' | beijing2008 | '); +----------------------------+
| trim(' | beijing2008 | ') | +----------------------------+
| | beijing2008 | | +----------------------------+
1 row in set (0.00 sec)

去掉字符串开头和结尾的空格
11,substring

mysql> select substring('beijing', 3, 8), substring('beijing', 3, 1); +----------------------------+----------------------------+
| substring('beijing', 3, 8) | substring('beijing', 3, 1) | +----------------------------+----------------------------+
| ijing | i | +----------------------------+----------------------------+
1 row in set (0.00 sec)

提取beijing从3开始后的8个字符串和从3开始后的一个字符串