sqlserver-中表的自链接

[luck] GO /****** Object: Table [dbo].[User_Activity] Script Date: 2017/8/20 15:14:32 ******/ SET ANSI_NULLS ON GO SET QUOTED_IDENTIFIER ON GO CREATE TABLE [dbo].[User_Activity]( [ID] [int] NOT NULL, [Actity] [nchar](100) NULL, [UserId] [int] NULL, CONSTRAINT [PK_User_Activity] PRIMARY KEY CLUSTERED ( [ID] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] ) ON [PRIMARY] GO

 

然后，是我们填充数据；

 

问题，现在我们要选出：

及：打球又约炮的同学；

有些同学的代码可能是这样的；

SELECT [ID]
      ,[Actity]
      ,[UserId]
  FROM [luck].[dbo].[User_Activity]
  
  where [Actity]=‘打球‘ and  [Actity]=‘约炮‘

---结果却是这样的

聪明一点的同学可能会这样写：

 

SELECT UserId
FROM [luck].[dbo].[User_Activity]
where [Actity]=‘打球‘ 
and  UserId in
(
  SELECT UserId
  FROM [luck].[dbo].[User_Activity]
  where [Actity]=‘约炮‘
 )

再或者：

SELECT M.UserId FROM(
SELECT UserId
FROM [luck].[dbo].[User_Activity]
where [Actity]=‘打球‘ ) M,
(
SELECT UserId
FROM [luck].[dbo].[User_Activity]
where [Actity]=‘约炮‘) N

WHERE M.UserId=N.UserId

 

到这一步，就已经很接近我们的表的自链接了；

 

SELECT M.UserId FROM [User_Activity] M,
                     [User_Activity] N

WHERE M.[Actity]=‘打球‘ AND N.[Actity]=‘约炮‘ AND
      M.UserId=N.UserId

 

 对啦，这个就是我们表的自连接的使用。

然后我们再来看一道实例：

 

sqlserver-中表的自链接

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