你真的会玩SQL吗？EXISTS和IN之间的区别
EXISTS和IN之间的区别
1.EXISTS只返回TRUE或FALSE，不会返回UNKNOWN。
2.IN当遇到包含NULL的情况，那么就会返回UNKNOWN。
当查询的列包含NULL时，NOT
EXISTS正常返回TRUE或FALSE。
而NOT IN可能返回空集,如下
1：val
IN(val1,val2,...,NULL)，永远不会返回FALSE，而是返回TRUE或UNKNOWN。
2：val NOT
IN(val1,val2,...,NULL)，永远不会返回TRUE，而是返回NOT TRUE或NOT UNKNOWN。
看个示例：
Test1表
select t.[name] from Test as t
where exists (select t1.orderid from Test1 as t1 where t1.[name]=t.[name])
返回 aaa,ccc,ddd
select t.[name] from Test as t
where t.[name] in (select t1.[] from Test1 as t1)
返回 aaa,ccc,ddd
select t.[name] from Test as t
where not exists (select t1.orderid from Test1 as t1 where t1.[name]=t.[name])
返回 bbb
select t.[name] from Test as t
where t.[name] not in (select t1.[name] from Test1 as t1)
返回空集
练习
以下对就返回哪三值？
答案
View Code
用例数据库文件 你真的会玩SQL吗？之逻辑查询处理阶段 文末
custid
companyname
----------- ----------------------------------------
21
Customer KIDPX
23
Customer WVFAF
33
Customer FVXPQ
36
Customer LVJSO
43
Customer UISOJ
51
Customer PVDZC
85
Customer ENQZT
参考SQL：
--answer:
select custid, companyname
from Sales.Customers as C
where EXISTS
(select *
from Sales.Orders as O
where O.custid =
C.custid
and O.orderdate >= '20070101'
and O.orderdate < '20080101')
and not EXISTS
(select *
from Sales.Orders as O
where O.custid =
C.custid
and O.orderdate >= '20080101'
and O.orderdate < '20090101');
