sql - MySQL Group By Hours

Question

I'm trying to get a report from my history table by hourly usage. history table is;

CREATE TABLE IF NOT EXISTS `history` (
`history_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`user_id` int(11) unsigned NOT NULL DEFAULT '0',
`created` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`history_id`),
KEY `user_id` (`user_id`),
KEY `created` (`created`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=1 ;

I want to group by HOUR and COUNT all records in a given date range.

Hours            |    Usage
------------------------------------
00:00-01:00      |      5
01:00-02:00      |      9
02:00-03:00      |      0 (or NULL)
03:00-04:00      |      20
...
...
...
22:00-23:00      |      11
23:00-00:00      |      1

I used a query something like this but it doesn't display all hours.

SELECT 

CASE 
  WHEN HOUR(created) BETWEEN 0 AND 1 THEN '00:00 - 01:00'
  WHEN HOUR(created) BETWEEN 1 AND 2 THEN '01:00 - 02:00'
  WHEN HOUR(created) BETWEEN 2 AND 3 THEN '02:00 - 03:00'
  WHEN HOUR(created) BETWEEN 3 AND 4 THEN '03:00 - 04:00'
  WHEN HOUR(created) BETWEEN 4 AND 5 THEN '04:00 - 05:00'
  WHEN HOUR(created) BETWEEN 5 AND 6 THEN '05:00 - 06:00'
  WHEN HOUR(created) BETWEEN 6 AND 7 THEN '06:00 - 07:00'
  WHEN HOUR(created) BETWEEN 7 AND 8 THEN '07:00 - 08:00'
  WHEN HOUR(created) BETWEEN 8 AND 9 THEN '08:00 - 09:00'
  WHEN HOUR(created) BETWEEN 9 AND 10 THEN '09:00 - 10:00'
  WHEN HOUR(created) BETWEEN 10 AND 11 THEN '10:00 - 11:00'
  WHEN HOUR(created) BETWEEN 11 AND 12 THEN '11:00 - 12:00'
  WHEN HOUR(created) BETWEEN 12 AND 13 THEN '12:00 - 13:00'
  WHEN HOUR(created) BETWEEN 13 AND 14 THEN '13:00 - 14:00'
  WHEN HOUR(created) BETWEEN 14 AND 15 THEN '14:00 - 15:00'
  WHEN HOUR(created) BETWEEN 15 AND 16 THEN '15:00 - 16:00'
  WHEN HOUR(created) BETWEEN 16 AND 17 THEN '16:00 - 17:00'
  WHEN HOUR(created) BETWEEN 17 AND 18 THEN '17:00 - 18:00'
  WHEN HOUR(created) BETWEEN 18 AND 19 THEN '18:00 - 19:00'
  WHEN HOUR(created) BETWEEN 19 AND 20 THEN '19:00 - 20:00'
  WHEN HOUR(created) BETWEEN 20 AND 21 THEN '20:00 - 21:00'
  WHEN HOUR(created) BETWEEN 21 AND 22 THEN '21:00 - 23:00'
  WHEN HOUR(created) BETWEEN 22 AND 23 THEN '22:00 - 23:00'
  WHEN HOUR(created) BETWEEN 23 AND 24 THEN '23:00 - 00:00'
END AS `Hours`,

COUNT(*) AS `usage`
FROM history
WHERE (created BETWEEN '2012-02-07' AND NOW())
GROUP BY 
  CASE 
    WHEN HOUR(created) BETWEEN 0 AND 1 THEN 1
    WHEN HOUR(created) BETWEEN 1 AND 2 THEN 2
    WHEN HOUR(created) BETWEEN 2 AND 3 THEN 3
    WHEN HOUR(created) BETWEEN 3 AND 4 THEN 4
    WHEN HOUR(created) BETWEEN 4 AND 5 THEN 5
    WHEN HOUR(created) BETWEEN 5 AND 6 THEN 6
    WHEN HOUR(created) BETWEEN 6 AND 7 THEN 7
    WHEN HOUR(created) BETWEEN 7 AND 8 THEN 8
    WHEN HOUR(created) BETWEEN 8 AND 9 THEN 9
    WHEN HOUR(created) BETWEEN 9 AND 10 THEN 10
    WHEN HOUR(created) BETWEEN 10 AND 11 THEN 11
    WHEN HOUR(created) BETWEEN 11 AND 12 THEN 12
    WHEN HOUR(created) BETWEEN 12 AND 13 THEN 13
    WHEN HOUR(created) BETWEEN 13 AND 14 THEN 14
    WHEN HOUR(created) BETWEEN 14 AND 15 THEN 15
    WHEN HOUR(created) BETWEEN 15 AND 16 THEN 16
    WHEN HOUR(created) BETWEEN 16 AND 17 THEN 17
    WHEN HOUR(created) BETWEEN 17 AND 18 THEN 18
    WHEN HOUR(created) BETWEEN 18 AND 19 THEN 19
    WHEN HOUR(created) BETWEEN 19 AND 20 THEN 20
    WHEN HOUR(created) BETWEEN 20 AND 21 THEN 21
    WHEN HOUR(created) BETWEEN 21 AND 22 THEN 22
    WHEN HOUR(created) BETWEEN 22 AND 23 THEN 23
    WHEN HOUR(created) BETWEEN 23 AND 24 THEN 24
END

It displays only if there are any records.

Hours            |    Usage
------------------------------------
00:00-01:00      |      5
01:00-02:00      |      9
23:00-00:00      |      1

Answer 1

Your existing query can be reduced to:

SELECT   CONCAT(HOUR(created), ':00-', HOUR(created)+1, ':00') AS Hours
  ,      COUNT(*) AS `usage`
FROM     history
WHERE    created BETWEEN '2012-02-07' AND NOW()
GROUP BY HOUR(created)

To display every hour, including those for which there is no data, you need to outer join with a table containing all the hours for which you want data. You can build such a table in your query using UNION:

SELECT   CONCAT(Hour, ':00-', Hour+1, ':00') AS Hours
  ,      COUNT(created) AS `usage`
FROM     history
  RIGHT JOIN (
                   SELECT  0 AS Hour
         UNION ALL SELECT  1 UNION ALL SELECT  2 UNION ALL SELECT  3
         UNION ALL SELECT  4 UNION ALL SELECT  5 UNION ALL SELECT  6
         UNION ALL SELECT  7 UNION ALL SELECT  8 UNION ALL SELECT  9
         UNION ALL SELECT 10 UNION ALL SELECT 11 UNION ALL SELECT 12
         UNION ALL SELECT 13 UNION ALL SELECT 14 UNION ALL SELECT 15
         UNION ALL SELECT 16 UNION ALL SELECT 17 UNION ALL SELECT 18
         UNION ALL SELECT 19 UNION ALL SELECT 20 UNION ALL SELECT 21
         UNION ALL SELECT 22 UNION ALL SELECT 23
  )      AS AllHours ON HOUR(created) = Hour
WHERE    created BETWEEN '2012-02-07' AND NOW() OR created IS NULL
GROUP BY Hour
ORDER BY Hour

However, the treatment of groups for which no data exists is really a matter for business logic that's best placed in your data access layer rather than in the database itself: indeed it should be trivial for your application to use a zero value whenever an hour is absent.