%代表任意多个字符
select * from user where username like '%huxiao';
select * from user where username like 'huxiao%';
select * from user where username like '%huxiao%';
_代表一个字符
select * from user where username like '_';
select * from user where username like '______';
select * from user where username like 'huxia_';
select * from user where username like 'h_xiao';
如果我就真的要查%或者_，怎么办呢？使用escape，转义字符后面的%或_就不作为通配符了，注意前面没有转义字符的%和_仍然起通配符作用
select username from gg_user where username like '%xiao\_%' escape '\';
select username from gg_user where username like '%xiao\%%' escape '\';
通常使用''来做转义字符,但是也可以使用其他的符号.
