SQL循环遍历，删除表里某一列是重复的数据，只保留一条。

DECLARE @tempId NVARCHAR(Max),
        @tempIDD uniqueidentifier         

WHILE EXISTS ( SELECT UserId
               FROM  Users Group By UserId having (count(*))>1 )

    BEGIN  

        SELECT TOP 1 @tempId = UserId
        FROM   (SELECT UserId
               FROM  Users Group By UserId having (count(*))>1 ) #cc

        Select Top 1 @tempIDD=Id FROM Users where UserId=@tempId

        DELETE  FROM Users  WHERE   [id] = @tempIDD
    END

上面这种方式，效率比较低。改进后的方式如下：

Delete From Users WHERE ID in (SELECT min（ID） FROM Users Group By UserId having (count（*))>1 )