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Old September 6th, 2005, 05:37 PM
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Default sorting duplicates in group by

I am looking for the best way to retrieve unique records using two fields as criteria for the duplicates. I have a table with xID and yID fields (along with a timestamp and primary id) and have been using a group by on the xID and yID which results in what I want....except, I want the latest item. It gives me the earlier item, even though I order by timestamp desc...see below

SELECT primaryID
FROM table
ORDER BY timestamp DESC

This is simplified, but is basically what I want. But I am getting no effect either way on the order by. I am using mySQL 4.

I am assuming group by doesn't allow this much control? I am using group by instead of distinct because I am selecting a bunch of other fields as well in my actual query. Maybe I should be using a subselect with a distinct?

This has always been a little gray for me...

Thanks for any help!

Old September 7th, 2005, 01:30 PM
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You need to work this from the "inside out".

Here's one way to do this. There are probably others.

Suppose you had a table in your database ('GroupTable') that contained the xID, yID, and maximum Timestamp values for each xID, yID combination.

This could be created by:
    INSERT GroupTable
    SELECT xID, yID, MAX(timestamp) as MaxDate
    FROM table
    GROUP BY xID, yID;
If this table actually did exist, you could JOIN your table to it and select
the primaryID for each resultant row:
SELECT primaryID
FROM table T1
INNER JOIN GroupTable T2
ON T1.xID = T2.xID
AND T1.yID = T2.yID
AND T1.Timestamp = T2.MaxDate;
In your case the GroupTable doesn't exist, but you can always materialize such a table by using a subquery:
SELECT primaryID
FROM table T1
    (SELECT xID, yID, MAX(timestamp) as maxdate
    FROM table
    GROUP BY xID, yID) T2
ON T1.xID = T2.xID
AND T1.yID = T2.yID
AND T1.Timestamp = T2.MaxDate;
Jeff Mason
Custom Apps, Inc.

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