Question

I am attempting to narrow results of an existing complex query based on conditional matches on multiple columns within the returned data set. I’ll attempt to simplify the data as much as possible here.

Assume that the following table structure represents the data that my existing complex query has already selected (here ordered by date):

+----+-----------+------+------------+
| id | remote_id | type | date       |
+----+-----------+------+------------+
|  1 |         1 | A    | 2011-01-01 |
|  3 |         1 | A    | 2011-01-07 |
|  5 |         1 | B    | 2011-01-07 |
|  4 |         1 | A    | 2011-05-01 |
+----+-----------+------+------------+

I need to select from that data set based on the following criteria:

• If the pairing of remote_id and type is unique to the set, return the row always
• If the pairing of remote_id and type is not unique to the set, take the following action:
  • Of the sets of rows for which the pairing of remote_id and type are not unique, return only the single row for which date is greatest and still less than or equal to now.

So, if today is 2011-01-10, I’d like the data set returned to be:

+----+-----------+------+------------+
| id | remote_id | type | date       |
+----+-----------+------+------------+
|  3 |         1 | A    | 2011-01-07 |
|  5 |         1 | B    | 2011-01-07 |
+----+-----------+------+------------+

For some reason I’m having no luck wrapping my head around this one. I suspect the answer lies in good application of group by, but I just can’t grasp it. Any help is greatly appreciated!

Answer 1

/* Rows with exactly one date - always return regardless of when date occurs */
SELECT id, remote_id, type, date
    FROM YourTable
    GROUP BY remote_id, type
    HAVING COUNT(*) = 1
UNION
/* Rows with more than one date - Return Max date <= NOW */
SELECT yt.id, yt.remote_id, yt.type, yt.date
    FROM YourTable yt
        INNER JOIN (SELECT remote_id, type, max(date) as maxdate
                        FROM YourTable
                        WHERE date <= DATE(NOW())
                        GROUP BY remote_id, type
                        HAVING COUNT(*) > 1) sq
             ON yt.remote_id = sq.remote_id
                 AND yt.type = sq.type
                 AND yt.date = sq.maxdate