SQL select MAX查找一个组的单个记录

Question

我有许多记录表示相同的信息，但与其他表的键不同。我需要按照它们的通用属性（Att1，Att2，Att3）对这些记录进行分组，然后选择所有通用记录的单个RecordID作为这些通用记录的主ID。只有当masterID为空时，该masterID才需要添加到它所代表的每个记录中。我无法更改数据结构，并且所有表值都是GUID。我试过MAX，但我没有得到分组。

TableA (Current State) 

RecordID   Att1   Att2  Att3  MasterID 
1     A   B  C   
2     A   B  C 
3     A   B  C 
4     D   E  F 
5     D   E  F 
6     D   E  F 
7     G   H  I   7 
8     G   H  I   7 
9     G   H  I   7 

更新：增加了预期结果。

TableA (Expected Result) 

RecordID   Att1   Att2  Att3  MasterID 
1     A   B  C  1 
2     A   B  C  1 
3     A   B  C  1 
4     D   E  F  4 
5     D   E  F  4 
6     D   E  F  4 
7     G   H  I  7 
8     G   H  I  7 
9     G   H  I  7 

Answer 1

试试这个：

UPDATE m 
SET 
    MasterID = s.RecordID 
FROM TableA AS m 
INNER JOIN (
    SELECT Att1, Att2, Att3, MIN(RecordID) AS RecordID   
    FROM TableA 
    GROUP BY Att1, Att2, Att3 
) AS s 
    ON m.Att1 = s.Att1 AND m.Att2 = s.Att2 AND m.Att3 = s.Att3 
WHERE 
    MasterID IS NULL 

Answer 2

如果你只需要选择，那么你可以使用下面的查询 -

select recordid , 
     Att1, 
     Att2, 
     Att3, 
     COALESCE(MasterID , (select min(a2.recordid) from tableA a2 
          where a2.Att1 = a1.Att1 
          and a2.Att2= a1.Att2 
          and a1.Att3 = a2.Att3)) 
from TableA a1 

Answer 3

呸，我不喜欢这种设置。你违反了良好的规范化实践，并且存在一些需要“外部”知识的固有假设。但是，你说你不能改变它，以便...

我认为以下应工作（厂商中立）：

UPDATE TableA as a SET masterId = (SELECT MIN(b.recordId) 
            FROM TableA as b 
            WHERE b.att1 = a.att1 
            AND b.att2 = a.att2 
            AND b.att3 = a.att3) 
WHERE masterId IS NULL 

---

编辑：

一旦启示，的GUID可以订购，但不能过去了MIN()或MAX() -

你有（至少）三个选项（是什么？）：

1. 把guid转换成可通过的类型（即 - SELECT MIN(CAST(b.recordId as CHAR(36)))）。然而，这可能是糟糕的表现，因为它会投射每一行（请参阅，这就是为什么它最好是内部ID为简单整数）。

2. 尝试此供应商中立的声明：

   UPDATE TableA as a SET masterId = (SELECT b.recordId 
               FROM TableA as b 
               LEFT JOIN TableA as c 
               ON c.att1 = b.att1 
               AND c.att2 = b.att2 
               AND c.att3 = b.att3 
               AND c.recordId < b.recordId 
               WHERE b.att1 = a.att1 
               AND b.att2 = a.att2 
               AND b.att3 = a.att3 
               AND b.recordId <= a.recordId 
               AND c.recordId IS NULL) 
   WHERE masterId IS NULL 
   

3. 还有用于SQL Server这更地道版本：

   UPDATE Updating 
   SET Updating.masterId = Origin.recordId 
   FROM TableA Updating 
   JOIN (TableA Origin 
        LEFT JOIN TableA Exclusion 
         ON Exclusion.att1 = Origin.att1 
          AND Exclusion.att2 = Origin.att2 
          AND Exclusion.att3 = Origin.att3 
          AND Exclusion.recordId < Origin.recordId) 
       ON Exclusion.recordId IS NULL 
       AND Origin.att1 = Updating.att1 
       AND Origin.att2 = Updating.att2 
       AND Origin.att3 = Updating.att3 
   WHERE Updating.masterId IS NULL 
   

   SQL Fiddle example

Answer 4

你可以使用交叉申请做出选择上。交叉应用基本上选择具有匹配的Att1，Att2和Att3的最低RecordID，并将RecordID作为名为MasteRID的列加入。

WITH temp as (SELECT RecordID, Att1, Att2, Att3, c.MasterID FROM dbo.TableA b 
    CROSS APPLY (SELECT TOP 1 RecordID as MasterID 
    FROM dbo.TableA a 
    WHERE a.Att1=b.Att1 
    AND a.Att2=b.Att2 AND a.Att3=b.Att3 
    ORDER BY RecordID) c) 

UPDATE TableA 
SET MasterID=t.MasterID 
FROM temp t 
WHERE RecordID=t.RecordID 

更新将使用相应的RecordID更新每个RecordID条目的MasterID列。