Question

I have a collection content that has four columns; id, timestamp, locationID, and authorID. Here is an example of my data; in production, this is tens of millions of rows in length.

id    timestamp              locationID   authorID
1     2012-03-01 11:52:00    1            1
2     2012-03-16 19:56:00    1            2
3     2012-04-02 11:26:00    2            1
4     2012-04-22 11:52:00    2            3
5     2012-05-19 09:48:00    2            2
6     2012-05-30 07:12:00    2            1
7     2012-06-04 19:17:00    1            2

I’d like to collect the list of authorIDs whose most recent content (ordered by timestamp) matched a specific locationID.

The correct values for a query of locationID = 2 would be: [ 1, 3 ], as authorID 1 and 3 were most recently ‘seen’ at locationID = 2, while authorID 2’s most recent content was at locationID 1.

I can certainly execute one query per authorID, but on production the authorID array has a length >100,000. This seems terribly inefficient (especially when each ‘subquery’ would be hitting this multi-million row content collection), and I’m looking for a better way to emerge this data from my dataset, ideally fast enough to be executed on a page render.

Answer 1

Something like this? This is from SQL Server, but I think it should work in mySQL as well.

DECLARE @locationId INT
SET @locationId = 2;

SELECT * 
FROM (SELECT AuthorId, Max(TimeStamp) as MaxTimeStamp
    FROM Content C
    WHERE LocationId = @locationId
    GROUP BY AuthorId) AS CBL
    LEFT JOIN Content AS C ON CBL.AuthorId = C.AuthorId
        AND C.TimeStamp > CBL.MaxTimeStamp
WHERE C.AuthorId IS NULL

For locationId = 2, it returns 1 and 3; and for locationId = 1, it returns 2

Per JW (thanks!), the correct mySql approach:

SET @locationId := 2;

SELECT * 
FROM (SELECT AuthorId, Max(TimeStamp) as MaxTimeStamp
    FROM Content C
    WHERE LocationId = @locationId
    GROUP BY AuthorId) AS CBL
    LEFT JOIN Content AS C ON CBL.AuthorId = C.AuthorId
        AND C.TimeStamp > CBL.MaxTimeStamp
WHERE C.AuthorId IS NULL