mysql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