mysql查询优化

标签：sql查询   必须   表数   sel   exist   等价   where   表数据   查询优化   

select * from a where id in (select id from b)

等价于:

for select id from b

for select 8 from a where a.id = b.id

当b表数据必须小于a表数据时,in优于exists

select * from a where exists (select 1 from b where b.id = a.id)

等价于:

for select * from a 

for select * from b where b.id = a.id 

防止exists优于in

mysql查询优化

标签：sql查询   必须   表数   sel   exist   等价   where   表数据   查询优化   

原文地址：https://www.cnblogs.com/saber-xi/p/13096018.html