SQL 2000 Left Join Top 1 of 0 to many关系

Question

This question has been asked multiple times on SO but all the answers refer to SQL 2005 or later (eg OUTER APPLY ) and we are still using SQL 2000 (for corporate reasons too complex to go into here!)

I have a table of Things and a table of Widgets with a 0 to Many relationship:

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' )

A standard LEFT OUTER JOIN returns the expected 4 rows

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 

However, I only want the newest Widget for each Thing, ie:

 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 

My starting point was:

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

But this is not valid because the parent t.ThingId does not exist in the sub query.

Can this be achieved using SQL 2000?

Answer 1

If (ThingId, WidgetId) combination is unique in table Widgets , then this will work correctly:

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 ;