从扩展行的 2 个表中选择数据

Question

我的问题是这样的：

我有两张桌子。一个承载真实数据，另一个用作备份。每当真实数据发生变化时，触发器就会将原始行复制到备份表中。

我需要的是：

我想选择所有原始的真实数据。这意味着真实数据表中从未更改过的所有条目以及第一次插入的备份表中的所有数据。

想象一下这些表格：

╔════════╦══════════╗ ╔══════════╦════════╦══════════╗
║ RealId ║ Numeric  ║ ║ BackupId ║ RealId ║ Numeric  ║
╠════════╬══════════╣ ╠══════════╬════════╬══════════╣
║      1 ║        3 ║ ║        1 ║      1 ║        7 ║
║      2 ║       19 ║ ║        2 ║      1 ║        9 ║
║      3 ║       24 ║ ║        3 ║      1 ║       14 ║
║      4 ║        5 ║ ║        4 ║      2 ║        2 ║
║      5 ║       23 ║ ║        5 ║      3 ║       13 ║
╚════════╩══════════╝ ║        6 ║      5 ║        9 ║
                      ║        7 ║      5 ║        4 ║
                      ╚══════════╩════════╩══════════╝

我的目标是创建一个返回如下内容的查询：

╔══════════╦════════╦══════════╗
║ BackupId ║ RealId ║ Numeric  ║
╠══════════╬════════╬══════════╣
║ 1        ║      1 ║        7 ║
║ 4        ║      2 ║        2 ║
║ 5        ║      3 ║       13 ║
║ NULL     ║      4 ║        5 ║
║ 6        ║      5 ║        9 ║
╚══════════╩════════╩══════════╝

如您所见，我总是希望从备份表中返回使用特定 Realid 创建的第一个条目。 （RealId 和 BackupId 始终是唯一的）

我想到的一种方法是

SELECT MIN(BackupId), RealId
FROM BackupTable
GROUP BY RealId

至于实桌，我想到了类似的东西

SELECT *
FROM real-table A
WHERE NOT EXISTS (
     SELECT * 
     FROM backup-table B
     WHERE B.RealId = A.RealId
)

但我实在想不出组合表格的正确方法。

Answer 1

使用窗口函数：

Create table #Real (RealID int, [Numeric] int)

Create table #Backup (BackupID int, RealID int, [Numeric] int)

Insert into #Real values(1, 3)
Insert into #Real values(2, 19)
Insert into #Real values(3, 24)
Insert into #Real values(4, 5)
Insert into #Real values(5, 23)

Insert into #Backup values (1, 1, 7)
Insert into #Backup values (2, 1, 9)
Insert into #Backup values (3, 1, 14)
Insert into #Backup values (4, 2, 2)
Insert into #Backup values (5, 3, 13)
Insert into #Backup values (6, 5, 9)
Insert into #Backup values (7, 5, 4)

Select distinct 
First_Value(b.[BackupID]) over (partition by r.RealID order by b.BackupID) as [BackupID]
    , r.RealID as [RealID]
    , isnull(First_Value(b.[Numeric]) over (partition by r.RealID order by b.BackupID), r.numeric) as [Numeric]
from #Real r 
left join #Backup b on r.realID = b.realid
order by r.[RealID]

Answer 2

我在 SO 上找到了这篇文章：

Select from table if record found in another table

这个问题的答案帮助我想出了一个这样的解决方案：

SELECT NULL AS BackupId, A.* FROM real-table A
WHERE NOT EXISTS (
    SELECT * 
    FROM backup-table B
    WHERE B.RealId = A.RealId
)

union all

Select C.* FROM backup-table C
INNER JOIN (
    SELECT MIN(BackupId) AS BackupId, 
           RealId
        FROM backup-table 
        GROUP BY RealId 
    ) D
on D.BackupId = C.BackupId

ORDER BY RealId Asc

但是，我想根据 cloudsafes 的答案对此进行测试，看看哪个性能更好。

Answer 3

对于 2012 年之前的任何人......

（* 现在编辑为假设所有 realID 都将存在于真实表中）

SELECT        
    firstB.minBU AS [first Backup]
    , R.realid
    , ISNULL(B.numeric, R.numeric) AS [original value]
FROM            
    (SELECT realid, MIN(backupid) AS minBU
        FROM test.[backup] AS backup_1 GROUP BY realid
    ) AS firstB 
    INNER JOIN
    test.[backup] AS B 
        ON firstB.realid = B.realid AND firstB.minBU = B.backupid 
    RIGHT OUTER JOIN
        test.real AS R ON firstB.realid = R.realid