select a,b from tab where a='dd' and b='cc'
与
select a,b from (select a,b from tab where a='dd') as tab1 where b='cc'
哪个性能高一点
与
select a,b from (select a,b from tab where a='dd') as tab1 where b='cc'
哪个性能高一点
要十万还是百万呀。