Counting messages per day (after 17:00 counts for next day) - mysql

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

Answer 2

Really, all you're doing is shifting the times by 7 hours. Something like this should work:
UPDATE stats s
SET count = (SELECT COUNT(msg_id) FROM messages m
WHERE m._date BETWEEN DATE_SUB(DATE_ADD(s._date, INTERVAL TIME_TO_SEC(m.time) SECOND), INTERVAL 7 HOUR)
AND DATE_ADD(DATE_ADD(s._date, INTERVAL TIME_TO_SEC(m.time) SECOND), INTERVAL 17 HOUR));
The basic idea is that it takes each date in your stats table, adjusts it by 7 hours, and looks for messages sent in that range. If you used a DATETIME column instead of separate DATE and TIME columns, you wouldn't need the extra DATE_ADD(..., TIME_TO_SEC) stuff.
There may be a better way to add a date and a time, I didn't see one with a quick look at the MySQL reference documents.
So all you'd need to do is insert a new row in the stats table with a 0 for the msgcount, and run the update command. If you only wanted to update a few days (since the message count probably isn't changing 6 days later) you just need a simple where clause on the update:
UPDATE stats s
SET ...
WHERE s._date BETWEEN '2012-04-03' AND '2012-04-08'