sqlserver-中表的自链接

sqlserver-表中的自链接

    为了方便，我对表进行简化，一个人能有多个活动；

USE [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；(这里查询的是，打球 and 约炮)  不是 打球 or 约炮

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：（这样写，是错的，因为，我们要的结果是，同时选择（参加了）了打球和约炮）

SELECT [ID]
      ,[Actity]
      ,[UserId]
  FROM [luck].[dbo].[User_Activity]
  where [Actity]='打球'
  UNION ALL
  SELECT [ID]
      ,[Actity]
      ,[UserId]
  FROM [luck].[dbo].[User_Activity]
  where [Actity]='约炮'

然后，我们再逐步的分析：（下面的代码）

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

结果；

方案一的实现（INTERSECT）

SELECT
      [UserId]
  FROM [luck].[dbo].[User_Activity]
  where [Actity]='打球' 
  --然后求一次交集；
  INTERSECT

SELECT 
      [UserId]
  FROM [luck].[dbo].[User_Activity]
  where [Actity]='约炮' 

方案二的实现（INNER JOIN）

SELECT
      A.[UserId]
  FROM [luck].[dbo].[User_Activity] AS A
  INNER JOIN (
SELECT 
       UserId,
      [Actity]
  FROM [luck].[dbo].[User_Activity]
 ) AS B
  ON A.UserId=B.UserId
  WHERE A.Actity='打球' and b.Actity='约炮'

方案三（子查询）

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'
 )

然后就一步步的，演变成我们的表自链接了~

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

然后，我们再来看一些其他的实例。。。。