连表查询,使用join与不使用有什么效率上的区别么?
例如:
select * from t1,t2 where t1.id = t2.p_id;
select * from t1 join t2 on (t1.id = t2.p_id);
至于left join/right join 我在条件后再加一个t1.xx or t2.xx is not null