Question

I have two MySQL tables: stats (left) and messages (right)

  +------------+---------+      +---------+------------+-----------+----------+
  |  _date     | msgcount|      | msg_id  | _date      | time      |  message |
  +------------+---------+      +----------------------+-----------+----------+
  | 2011-01-22 |  2      |      |   1     | 2011-01-22 |  06:23:11 | foo bar  |
  | 2011-01-23 |  4      |      |   2     | 2011-01-22 |  15:17:03 | baz      |
  | 2011-01-24 |  0      |      |   3     | 2011-01-22 |  17:05:45 | foobar   |
  | 2011-01-25 |  1      |      |   4     | 2011-01-22 |  23:58:13 | barbaz   |
  +------------+---------+      |   5     | 2011-01-23 |  00:06:32 | foo foo  |
                                |   6     | 2011-01-23 |  13:45:00 | bar foo  |
                                |   7     | 2011-01-25 |  02:22:34 | baz baz  |
                                +---------+------------+-----------+----------+

I filled in stats.msgcount, but in reality it is still empty. I’m looking for a query way to:

• count the number of messages for every stats._date (notice the zero msgcount on 2011-01-25)
• messages.time is in 24-hour format. All messages AFTER 5 o’clock (17:00:00) should be counted for the next day (notice msg_id 3 and 4 count for 2011-01-23)
• update stats.msgcount to hold all counts

I’m especially concerned about the “later than 17:00:00 count for next day” part. Is this possible in (My)SQL?

Answer 1

You could use:

UPDATE stats LEFT JOIN
  ( SELECT date(addtime(_date,time) + interval 7 hour) as corrected_date, 
           count(*) as message_count
    FROM messages
    GROUP BY corrected_date ) mc
ON stats._date = mc.corrected_date
SET stats.msgcount = COALESCE( mc.message_count, 0 )

However this query requires dates you are interested in to be in the stats table already, if you don’t have them make _date primary or unique key if its not yet and use:

INSERT IGNORE INTO stats(_date,msgcount)
SELECT date(addtime(_date,time) + interval 7 hour) as corrected_date,
       count(*) as message_count
FROM messages
GROUP BY corrected_date