1、AND比OR的优先级高

--Y
select 'Y' from dual where 1=2 and 1=2 or 1=1;


--Y
select 'Y' from dual where (1=2 and 1=2) or 1=1;


--No value
select 'Y' from dual where 1=2 and (1=2 or 1=1);

如下sql：

SELECT COUNT(u.user_id) FROM user_info u WHERE u.user_name='1' AND u.password ='123456' or 1=1 ; （有数据）

SELECT COUNT(u.user_id) FROM user_info u WHERE u.user_name='1' or 1=1AND u.password ='123456' ; （0）