IN 和EXISTS

IN 是把外表和内表作hash连接，而EXISTS是对外表作loop循环，每次loop循环再对内表进行查询。EXISTS比in效率高的说法是不正确的。如果查询的两个表大小相当，那么用in和EXISTS差别不大。

如果连个表中一个较大，一个是小表，则子查询表达的用EXISTS，子查询表小的用in。

例如：表Athor(小表),表Winner（大表）

SELECT * FROM Athor AthorCode in (select AthorCode? from Winner)

效率低，用到了Athor的索引

?

SELECT * FROM Athor EXISTS(select AthorCode? from Winner WHERE Athor.AthorCODE=Winner.AthorCODE)

效率高，用到了Winner 的索引

not in 和not exists

如果查询语句使用了not in 那么内外表都进行了全表扫描，没有用到索引；而not exists的子查询依然能用到表上的索引。所以无论那个表大，用not exists都比not in 要快。

NOT IN

select AthorCode,AthorName
from Athor
where AthorCode not in (
select AthorCode
from Winner)

NOT EXISTS

select AthorCode,AthorName
from Athor AS A
where? NOT EXISTS(
select AthorCode
from Winner? AS B
where A.AthorCode=B.AthorCode)

LEFT JOIN

select A.AthorCode,AthorName
from Athor AS A Left join Winner B
ON A.AthorCode=B.AthorCode
WHERE B.WinnerDate IS NOT NULL