Home/ Questions/Q 138999
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-11T07:21:24+00:00

I have two tables: TableA ( ID [int, pk], Name [string]) and TableB (ID

  • 0

I have two tables: TableA (ID [int, pk], Name [string]) and TableB (ID [int, pk], TableA_ID [int, fk], Name [string], DateStamp [datetime (dd/mm/yyyy hh:MM:ss)]). There is a one-to-many relationship between TableA and TableB.

An inner join on the two tables will give me the following results:

 TableA.ID, TableA.Name, TableB.Name, TableB.DateStamp         1,    'File A', 'Version 1', 01/01/2009 15:00:00         1,    'File A', 'Version 2', 05/01/2009 08:15:00         1,    'File A', 'Version 3', 06/01/2009 19:33:00         2,    'File B', 'Version 1', 03/01/2009 09:10:00         2,    'File B', 'Version 2', 20/01/2009 20:00:00         3,    'File C', 'Version 1', 01/01/2009 17:00:00

What I actually want is the following (each row from TableA and the last matching row from TableB):

 TableA.ID, TableA.Name, TableB.Name, TableB.DateStamp         1,    'File A', 'Version 3', 06/01/2009 19:33:00         2,    'File B', 'Version 2', 20/01/2009 20:00:00         3,    'File C', 'Version 1', 01/01/2009 17:00:00

This is the query that I’m using to achieve this:

SELECT ta.ID, ta.Name, tb.Name, tb.DateStamp FROM TableA ta INNER JOIN TableB tb ON ta.ID = tb.TableA_ID WHERE tb.ID IN (  SELECT TOP 1 tb2.ID   FROM TableB tb2   WHERE tb2.TableA_ID = ta.ID   ORDER BY tb2.DateStamp DESC)

That works but my gut feeling is that I’m not doing this in the ‘best way’. It looks like it is a candidate for an aggregation query (i.e. groupby) but I didn’t have any luck with that. In the end I always had to use a subquery to get the row I’m after in TableB.

Any help much appreciated.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. No, there is no requirement to do GROUP BY here, this should be solved through a correlated sub-query:

    SELECT   TableA.ID,    TableA.Name,    TableB.Name,    TableB.DateStamp FROM   TableA   INNER JOIN TableB ON      TableA.ID = TableB.TableA_ID     AND TableB.DateStamp = (       SELECT MAX(DateStamp)        FROM TableB       WHERE TableA_ID = TableA.ID     )

    An additional GROUP BY is only necessary if you have more than one record in TableB with equal TableA_ID and equal DateStamp.

    For the specific example you’ve shown, a GROUP BY query happens to produce the correct result. It’s still wrong, because the correct result is more a side-effect in this situation.

    SELECT   TableA.ID,    TableA.Name,    MAX(TableB.Name) Max_TableBName,    MAX(TableB.DateStamp) Max_TableBDateStamp FROM   TableA   INNER JOIN TableB ON TableA.ID = TableB.TableA_ID GROUP BY   TableA.ID,    TableA.Name

    This relies on the coincidence that MAX(TableB.Name) is in fact the value you want to get out, and it is aligned with MAX(TableB.DateStamp). But since this correlation is a mere accident, the GROUP BY query is wrong.

    • 0