Question

Consider the following table:

msg_id  _time
-------------
1       13:32
2       13:56
3       14:05
4       15:00
5       15:17
6       15:28
7       16:40

I’m currently aggregating the number of messages per hour, as follows:

SELECT HOUR(_time) AS hour, COUNT(msg_id) AS cnt FROM messages GROUP BY hour 
ORDER BY hour

This results in a resultset as follows:

hour  cnt
---------
13    2
14    1
15    3
16    1

What I want, however, is the count per half hour:

hour   cnt
----------
13:30  2
14:00  1
14:30  0
15:00  3
15:30  0
16:00  0
16:30  1

What is the query making this possible? If necessary, I guess I could mkae a table with the half-hour times (00:00, 00:30, 01:00, 01:30, … 23:00, 23:30), but prefer not too. (And I still wouldn’t know how to use that table either).

Any help is greatly appreciated 🙂

Answer 1

you can use case when

SELECT COUNT(msg_id) AS cnt, CONCAT(CAST(HOUR(_TIME) AS CHAR(2)), ':', (CASE WHEN MINUTE(_time) < 30 THEN '00' ELSE '30' END)) AS hour
FROM messages
GROUP BY CONCAT(CAST(HOUR(_TIME) AS CHAR(2)), ':', (CASE WHEN MINUTE(_time) < 30 THEN '00' ELSE '30' END))
ORDER BY hour