[英]SQL 2000 Left Join Top 1 of 0 to many relationship
這個問題在 SO 上被問過多次,但所有的答案都是指 SQL 2005 或更高版本(例如OUTER APPLY
),我們仍在使用 SQL 2000(由於公司原因太復雜,無法進入這里!)
我有一個Things
表和一個具有 0 到多關系的Widgets
表:
CREATE TABLE Things ( ThingId INT, ThingName VARCHAR(50) )
CREATE TABLE Widgets ( WidgetId INT, ThingId INT, WidgetName VARCHAR(50) )
INSERT INTO Things VALUES ( 1, 'Thing 1' )
INSERT INTO Things VALUES ( 2, 'Thing 2' )
INSERT INTO Things VALUES ( 3, 'Thing 3' )
INSERT INTO Widgets VALUES ( 1, 2, 'Thing 2 Widget 1' )
INSERT INTO Widgets VALUES ( 2, 2, 'Thing 2 Widget 2' )
INSERT INTO Widgets VALUES ( 3, 3, 'Thing 3 Widget 1' )
標准的LEFT OUTER JOIN
返回預期的 4 行
SELECT * FROM Things t LEFT OUTER JOIN Widgets w ON t.ThingId = w.ThingId
ThingId | ThingName | WidgetId | ThingId | WidgetName
---------+-----------+----------+---------+------------------
1 | Thing 1 | NULL | NULL | NULL
2 | Thing 2 | 1 | 2 | Thing 2 Widget 1
2 | Thing 2 | 2 | 2 | Thing 2 Widget 2
3 | Thing 3 | 3 | 3 | Thing 3 Widget 1
但是,我只想要每個事物的最新小部件,即:
ThingId | ThingName | WidgetId | ThingId | WidgetName
---------+-----------+----------+---------+------------------
1 | Thing 1 | NULL | NULL | NULL
2 | Thing 2 | 2 | 2 | Thing 2 Widget 2
3 | Thing 3 | 3 | 3 | Thing 3 Widget 1
我的出發點是:
SELECT * FROM Things t LEFT OUTER JOIN (SELECT TOP 1 * FROM Widgets subw WHERE subw.ThingId = t.ThingId ORDER BY subw.WidgetId DESC) w ON t.ThingId = w.ThingId
但這無效,因為子查詢中不存在父t.ThingId
。
這可以使用 SQL 2000 實現嗎?
如果(ThingId, WidgetId)
組合在表Widgets
是唯一的,那么這將正常工作:
SELECT t.*, w.*
FROM
dbo.Things AS t
LEFT OUTER JOIN
( SELECT ThingId, MAX(WidgetId) AS WidgetId
FROM dbo.Widgets
GROUP BY ThingId
) AS
subw
ON subw.ThingId = t.ThingId
LEFT OUTER JOIN
dbo.Widgets AS w
ON w.ThingId = subw.ThingId
AND w.WidgetId = subw.WidgetId ;
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