## 字符串函数 char_length()：判断字符串的字符数. ~~~ select char_length('哈哈'); ~~~ 结果: ~~~ +-----------------------+ | char_length('哈哈') | +-----------------------+ | 2 | +-----------------------+ ~~~ * * * * * length() :判断字符串的字节数（与字符集); ~~~ select length('哈哈'); ~~~ 结果: ~~~ +------------------+ | length('哈哈') | +------------------+ | 6 | +------------------+ ~~~ * * * * * concat()：连接字符串. ~~~ select concat('hello','world'); ~~~ 结果: ~~~ +-------------------------+ | concat('hello','world') | +-------------------------+ | helloworld | +-------------------------+ ~~~ * * * * * instr()：判断字符在目标字符串中是否存在，存在返回其位置，不存在返回0. ~~~ select instr('helloworld','hello'); ~~~ 结果: ~~~ +-----------------------------+ | instr('helloworld','hello') | +-----------------------------+ | 1 | +-----------------------------+ ~~~ * * * * * lcase()：全部小写. ~~~ select lcase('HELLO'); ~~~ 结果: ~~~ +----------------+ | lcase('HELLO') | +----------------+ | hello | +----------------+ ~~~ * * * * * left()：从左侧开始截取，直到指定位置（位置如果超过长度，截取所有). ~~~ select left('hellowworld',5); ~~~ 结果: ~~~ +-----------------------+ | left('hellowworld',5) | +-----------------------+ | hello | +-----------------------+ ~~~ * * * * * ltrim()：消除左边对应的空格. ~~~ select ltrim(' hello'); ~~~ 结果: ~~~ +-------------------+ | ltrim(' hello') | +-------------------+ | hello | +-------------------+ ~~~ * * * * * mid()：从中间指定位置开始截取，如果不指定截取长度，直接到最后. ~~~ select mid('helloworld',5); ~~~ 结果: ~~~ +---------------------+ | mid('helloworld',5) | +---------------------+ | oworld | +---------------------+ ~~~