在sql中使用exists替换in查询时要注意使用exists时一定要关联主查询和子查询的关联不然查询会得不到相应的结果如下语句：
语句一使用in查询：
select realname from Users where Users.UserId in
(select Gallery.Galleries.CreatorId from Gallery.Galleries group by Gallery.Galleries.CreatorId
having COUNT(Gallery.Galleries.CreatorId)>1 ) Order by UserId
语句二使用exists查询：
select realname from Users where exists
(select Gallery.Galleries.CreatorId from Gallery.Galleries group by Gallery.Galleries.CreatorId
having COUNT(Gallery.Galleries.CreatorId)>1 ) Order by UserId
乍一看没有错误，但是语句二忘记了主查询和子查询的主键的关联，导致把主查询的所有内容查出来。
语句二的正确写法应该是：
select * from Gallery.Galleries as A where exists (select A .CreatorId from Gallery.Galleries as B
Where A.CreatorId = B.CreatorId group by B.CreatorId having COUNT(B.CreatorId)>1)Order by CreatorId
注：exists是boolean运算的只要字查询的结果集有一条数据结果就为真where后的条件就为真所以第二条查询语句等价于
select realname from Users where 1=1 Order by UserId（如果子查询有一条数据被查出）
