SQL 服务器将前两行合并为两列

Question

我有两个表：

Table User:

Id, Name, Email,...

Table Images:

UserId, Priority, Path,...

我想创建一个用户视图，其中包含每个具有最高优先级的用户的两张图片，结果如下：

User.Id, User.Name, User.Email, Image1, Image2
-------------------------------------------------

我想，如果用户没有图片，Image1和Image2两列都会是NULL，如果只有一张图片，那么Image1 将被设置并且 Image2 将是 NULL.

我怎样才能最有效地做到这一点？我的意思是数据库性能。

Answer 1

使用这些示例架构/数据作为模型：

CREATE TABLE #User (Id INT, Name VARCHAR(MAX), Email VARCHAR(MAX))
CREATE TABLE #Images (UserId INT, [Priority] INT, [Path] VARCHAR(MAX))
INSERT INTO #User VALUES
(1, 'Bob', 'bob@foo.com'),
(2, 'Jim', 'jim@foo.com')

INSERT INTO #Images VALUES
(1, 10, 'path1'),
(1, 5, 'path2'),
(1, 7, 'path3')

你可以使用 PIVOT:

SELECT UserId, [1] AS PathOfImage1, [2] AS PathOfImage2
FROM 
(SELECT UserId, [Path], 
        ROW_NUMBER() OVER (PARTITION BY UserId ORDER BY [Priority] DESC) AS rn
 FROM #Images ) AS source
 PIVOT 
 (
   MAX([Path])
   FOR rn IN ([1], [2])
 ) AS pvt

要在单个记录中为每个用户获取 top 两张图片：

UserId  PathOfImage1    PathOfImage2
------------------------------------
1       path1           path3

请注意，Id=2 的用户没有出现在上面的结果集中，因为他没有任何相关图片。

您现在可以对上述查询生成的 table 表达式执行 OUTER APPLY：

SELECT u.Id, u.Name, Images.PathOfImage1, Images.PathOfImage2
FROM #User u
OUTER APPLY (
   SELECT UserId, [1] AS PathOfImage1, [2] AS PathOfImage2
   FROM (
      SELECT UserId, [Path], 
             ROW_NUMBER() OVER (PARTITION BY UserId ORDER BY [Priority] DESC) AS rn
      FROM #Images ) AS source
   PIVOT 
   (
      MAX([Path])
      FOR rn IN ([1], [2])
   ) AS pvt
   WHERE UserId = u.Id
) Images

为了得到想要的输出：

Id  Name    PathOfImage1    PathOfImage2
----------------------------------------
1   Bob     path1           path3
2   Jim     NULL            NULL

编辑：

如果应从 Image table 中提取多个列，则 PIVOT 必须替换为旧式条件聚合，因为 SQL 服务器不在 PIVOT:

上支持多个聚合

SELECT t.UserId, 
       [PathOfImage1] = MAX(CASE WHEN rn = 1 THEN [Path] ELSE NULL END),
       [PathOfImage2] = MAX(CASE WHEN rn = 2 THEN [Path] ELSE NULL END),
       [PriorityOfImage1] = MAX(CASE WHEN rn = 1 THEN [Priority] ELSE NULL END),
       [PriorityOfImage2] = MAX(CASE WHEN rn = 2 THEN [Priority] ELSE NULL END)
FROM (
  SELECT UserId, [Path], [Priority],
         ROW_NUMBER() OVER (PARTITION BY UserId ORDER BY [Priority] DESC) AS rn
  FROM #Images ) t
GROUP BY t.UserId

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