关系运算符：or、and的优先级问题

and的优先级大于or

如：

①select * from tablename where condition1 or condition2 and condition3

其实相当于

select * from tablename where condition1 or (condition2 and condition3)

②select * from tablename  where condition1 and condition 2 or condition3 and condition4

其实相当于

select * from tablename  where (condition1 and condition 2) or (condition3 and condition4)

注意：

   在不确定执行顺序的时候，最好加上括号，括号的优先级最高，可读性也强