Question

I have a table (table1):

ID   INSERTDATE    ISSUE    SOURCE
1    2011-12-01    A        EMAIL
2    2011-12-02    B        POST
3    2011-12-03    C        MEETING
4    2011-12-04    B        INTERNET

I would like to get the latest SOURCE for each issue…

SELECT * FROM table1 GROUP BY SOURCE ORDER BY ISSUE ASC

Results in

ID   INSERTDATE    ISSUE    SOURCE
1    2011-12-01    A        EMAIL
2    2011-12-02    B        POST
3    2011-12-03    C        MEETING

But how can I get the following, so that issue B has the latest source?

ID   INSERTDATE    ISSUE    SOURCE
1    2011-12-01    A        EMAIL
4    2011-12-04    B        INTERNET
3    2011-12-03    C        MEETING

Answer 1

You need an aggregate and a self join.
This is standard SQL without relying on the MySQL group by extensions (which are rubbish)

SELECT
    t.*
FROM
    ( -- get most recent ID per issue
    SELECT MAX(ID) AS MaxID, ISSUE
    FROM table1
    GROUP BY ISSUE
    ) t2
    JOIN  -- join back to get the whole row
    table1 t ON t2.MaxID = t.ID

Edit, after comment. Assumes one change per day only

SELECT
    t.*
FROM
    ( -- get most recent INSERTDATE per issue
    SELECT MAX(INSERTDATE) AS MaxINSERTDATE, ISSUE
    FROM table1
    GROUP BY ISSUE
    ) t2
    JOIN  -- join back to get the whole row
    table1 t ON t2.MaxINSERTDATE = t.ID