mysql查詢優化

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

等價於:

for select id from b

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

當b表數據必須小於a表數據時,in優於exists

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

等價於:

for select * from a 

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

防止exists優於in