not exists相当于not in .并且not exists比not in 效率高

比如表A主键id与表B的外键a_id关联,要在A表中查出id不等于B表中所有a_id的记录:
not in 写法: select * from A where id not in (select a_id from B where a_id = A.id )
not exists写法: select * from A where id not exists (select 1 from B where a_id = A.id)