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

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