Question

I have a very weird thing to do and simply don’t know how. This is my table and its data:

+------+-----------+-------------+-------+------------+
| uid  | entity_id | entity_type | state | created    |
+------+-----------+-------------+-------+------------+
| 3913 |      1105 |           1 |     0 | 1340617072 |
| 3913 |      1105 |           1 |     1 | 1340617042 |
| 3913 |      3930 |           1 |     0 | 1340617071 |
| 3913 |      3930 |           1 |     1 | 1340617036 |
+------+-----------+-------------+-------+------------+

created holds the timestamp for which an action has been made, state is the state of the action (followed or unfollowed), uid is who makes the action and entity_id is the entity ID which is being followed/unfollowed.

I’m trying to do a query which gets me all the entity_id which the current user follows. First I was using:

SELECT entity_id FROM follow_objects WHERE uid=? AND entity_type=?

However I noticed that if I follow someone and then unfollow them, I still get a result for that person, because there are 2 entries for it (with state=1 and then state=0).

So I need to get the rows with the newest created column, which means the action is the most recent one. So I’ve tried:

SELECT entity_id FROM follow_objects WHERE uid=? AND entity_type=? group by entity_id HAVING created=MAX(created)

Which returns only rubbish data. I’m out of ideas. Any help?

Answer 1

You need to select on a group-wise maximum. You must wrap the maximum in a subselect and join the table on itself where the entity_ids are equal and where the subselect’s max timestamp equals the created timestamp. The result is that you’ll get the top row data per entity_id group:

SELECT
    a.entity_id,
    b.state,
    b.created
FROM
    (
        SELECT entity_id, MAX(created) AS max_created
        FROM follow_objects
        WHERE uid = ? AND entity_type = ?
        GROUP BY entity_id
    ) a
INNER JOIN
    follow_objects b ON
        a.entity_id = b.entity_id AND
        a.max_created = b.created