Question

I am kind of stuck..

I have two tables

mysql > EXPLAIN event;
+-------------+--------------+------+-----+---------------------+-----------------------------+
| Field       | Type         | Null | Key | Default             | Extra                       |
+-------------+--------------+------+-----+---------------------+-----------------------------+
| id          | int(11)      | NO   | PRI | NULL                | auto_increment              |
| name        | varchar(100) | NO   | MUL |                     |                             |
| extUrl      | varchar(192) | NO   |     |                     |                             |
+-------------+--------------+------+-----+---------------------+-----------------------------+

One event can have many

mysql> explain eventDate;
+----------------+--------------+------+-----+---------------------+-----------------------------+
| Field          | Type         | Null | Key | Default             | Extra                       |
+----------------+--------------+------+-----+---------------------+-----------------------------+
| id             | int(11)      | NO   | PRI | NULL                | auto_increment              |
| category_id    | int(11)      | NO   | MUL | 0                   |                             |
| event_id       | int(11)      | NO   | MUL | 0                   |                             |
| location_id    | int(11)      | NO   | MUL | 0                   |                             |
| name           | varchar(100) | NO   |     |                     |                             |
| eventDate      | timestamp    | NO   |     | 0000-00-00 00:00:00 |                             |
+----------------+--------------+------+-----+---------------------+-----------------------------+

I’d like to query the event by id, with the location_id of the next upcoming eventDate and I am stuck how to form that query

I currently have this

 SELECT
      e.id,
      d.location_id,
      d.eventDate
 FROM
      event e
          INNER JOIN eventDate d ON d.event_id = e.id AND d.eventDate >= CURRENT_TIMESTAMP
 WHERE
      e.id = 5107
 GROUP BY
      e.id
 ORDER BY
      d.eventDate ASC

Now this returns a result, but I GROUP BY event, so I am unsure how to ensure that really the next eventDate is selected. Also I didn’t use any aggregate function ( group by one, aggregate many ), which doesn’t seem to be a clean and clearly defined result requirement – MySQL is randomly selecting one of the eventDate rows now isn’t it?

Answer 1

This should always give you the first upcoming event:

 SELECT
      e.id,
      d.location_id,
      d.eventDate
 FROM
      event e
          INNER JOIN eventDate d ON d.event_id = e.id
 WHERE
      e.id = 5107
      AND
      d.id = (SELECT d1.id FROM eventDate d1 WHERE d1.event_id = 5107 AND d1.eventDate >= CURRENT_TIMESTAMP ORDER BY d1.eventDate ASC LIMIT 1)

But the better way is to add LIMIT 1 to your current query and removing the GROUP BY:

 SELECT
      e.id,
      d.location_id,
      d.eventDate
 FROM
      event e
          INNER JOIN eventDate d ON d.event_id = e.id AND d.eventDate >= CURRENT_TIMESTAMP
 WHERE
      e.id = 5107
 ORDER BY
      d.eventDate ASC
 LIMIT 1