SQL：在没有 PIVOT 的情况下有效地将列连接到行

Question

我有以下两个table。每个Object可以有0-4个对应的Attributes，存储在Attributestable.

对象

ID	Title
1	Alpha
2	Beta
3	Gamma

属性

ID	ObjectID	AttributeName	AttributeValue
1	1	K1	12345
2	1	K2	23456
3	2	K1	34567
4	3	K2	45678
5	3	K3	56789
6	3	K4	67890

现在我想要以下结果，其中为每个可能的属性创建一个包含相应属性或 NULL 的列。

属性对象

ID	Title	K1	K2	K3	K4
1	Alpha	12345	23456
2	Beta	34567
3	Gamma		45678	56789	67890

我想出了两个查询，它们产生了想要的结果。我被迫使用的数据库 不支持 PIVOT 子句 。在这种情况下，对于想要的结果，以下两个中哪个更有效？

查询基于SELECT

SELECT ID, Title,
    (SELECT AttributeValue FROM Attributes WHERE AttributeName = "K1" AND ObjectID = Objects.ID) AS K1,
    (SELECT AttributeValue FROM Attributes WHERE AttributeName = "K2" AND ObjectID = Objects.ID) AS K2,
    (SELECT AttributeValue FROM Attributes WHERE AttributeName = "K3" AND ObjectID = Objects.ID) AS K3,
    (SELECT AttributeValue FROM Attributes WHERE AttributeName = "K4" AND ObjectID = Objects.ID) AS K4
FROM Objects;

基于 JOIN 的查询

SELECT Objects.ID, Objects.Title, Q1.K1, Q2.K2, Q3.K3, Q4.K4
FROM (((Objects 
    LEFT JOIN (SELECT ObjectID, AttributeValue AS K1 FROM Attributes WHERE AttributeName = "K1")
        AS Q1 ON Objects.ID = Q1.ObjectID)
    LEFT JOIN (SELECT ObjectID, AttributeValue AS K2 FROM Attributes WHERE AttributeName = "K2")
        AS Q2 ON Objects.ID = Q2.ObjectID)
    LEFT JOIN (SELECT ObjectID, AttributeValue AS K3 FROM Attributes WHERE AttributeName = "K3")
        AS Q3 ON Objects.ID = Q3.ObjectID)
    LEFT JOIN (SELECT ObjectID, AttributeValue AS K4 FROM Attributes WHERE AttributeName = "K4")
        AS Q4 ON Objects.ID = Q4.ObjectID;

Answer 1

使用 Pivot 和 Join

SELECT ID,Title, 
  K1,K2,K3,K4  
FROM  
(
  SELECT o.ID,Title,AttributeName, AttributeValue   
  FROM Objects o
  JOIN Attributes a ON o.ID = a.ObjectID
) AS SourceTable  
PIVOT  
(  
  AVG(AttributeValue)  
  FOR AttributeName IN (K1,K2,K3,K4)  
) AS PivotTable;

演示在 db<>fiddle

Answer 2

我提出了另一种替代方法，使用 IF 或 CASE 和 GROUP BY 似乎更有效，因此您无需调整任何内容。因为我不知道你用这些值做什么，所以我使用了 SUM。请参阅以下内容：

SELECT Title,
       SUM(IF(AttributeName = 'K1', AttributeValue, 0)) AS K1,
       SUM(IF(AttributeName = 'K2', AttributeValue, 0)) AS K2,
       SUM(IF(AttributeName = 'K3', AttributeValue, 0)) AS K3,
       SUM(IF(AttributeName = 'K4', AttributeValue, 0)) AS K4
FROM AttributedObjects AS AO
INNER JOIN Attributes AS A ON AO.ObjectID = O.ID
GROUP BY Title

Answer 3

使用conditional aggregation如下：

select o.id, o.title,
    Max(case when AttributeName='K1' then AttributeValue end) K1,
    Max(case when AttributeName='K2' then AttributeValue end) K2,
    Max(case when AttributeName='K3' then AttributeValue end) K3,
    Max(case when AttributeName='K4' then AttributeValue end) K4
from objects o
join attributes a on a.ObjectID = o.id
group by o.Id, o.title

SQL：在没有 PIVOT 的情况下有效地将列连接到行

SQL: Join columns to rows without PIVOT efficiently

sql

database

select

pivot

join

对象

属性

属性对象

查询基于SELECT

基于 JOIN 的查询