%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.