MySQL: filling empty fields with zeroes when using GROUP BY

Question

I've got MySQL table

CREATE TABLE cms_webstat (
    ID int NOT NULL auto_increment PRIMARY KEY,
    TIMESTAMP_X timestamp DEFAULT CURRENT_TIMESTAMP,
    # ... some other fields ...
)

which contains statistics about site visitors.
For getting visits per hour I use

SELECT
    hour(TIMESTAMP_X) as HOUR
    , count(*) AS HOUR_STAT
FROM cms_webstat
GROUP BY HOUR
ORDER BY HOUR DESC

which gives me

| HOUR | HOUR_STAT |
|  24  |    15     |
|  23  |    12     |
|  22  |    9      |
|  20  |    3      |
|  18  |    2      |
|  15  |    1      |
|  12  |    3      |
|   9  |    1      |
|   3  |    5      |
|   2  |    7      |
|   1  |    9      |
|   0  |    12     |

And I'd like to get following:

| HOUR | HOUR_STAT |
|  24  |    15     |
|  23  |    12     |
|  22  |    9      |
|  21  |    0      |
|  20  |    3      |
|  19  |    0      |
|  18  |    2      |
|  17  |    0      |
|  16  |    0      |
|  15  |    1      |
|  14  |    0      |
|  13  |    0      |
|  12  |    3      |
|  11  |    0      |
|  10  |    0      |
|   9  |    1      |
|   8  |    0      |
|   7  |    0      |
|   6  |    0      |
|   5  |    0      |
|   4  |    0      |
|   3  |    5      |
|   2  |    7      |
|   1  |    9      |
|   0  |    12     |

How should I modify the query to get such result (with one mysql query, without creating temporary tables)?
Is it possible to get such result with one MySQL query?

Answer 1

Create another table with a single column,

CREATE TABLE hours_list (
    hour int NOT NULL PRIMARY KEY
)

Fill it with all 24 hours.

Then do a join on that table to fill in the zeroes.

SELECT
    hs.hour as HOUR, COUNT(ws.ID) AS HOUR_STAT
FROM hours_list hs 
LEFT JOIN cms_webstat ws ON hs.hour = hour(ws.TIMESTAMP_X)
GROUP BY hs.hour
ORDER BY hs.hour DESC

Answer 2

This is just the 'why it is not returning` part. Marcus' answer covers the 'how to' part.

The SQL

SELECT 
    hour(TIMESTAMP_X) as HOUR 
    , count(*) AS HOUR_STAT 
FROM cms_webstat 
GROUP BY HOUR 
ORDER BY HOUR DESC 

gets the count of the records per hour, for the timestamps present in the table

It does not give the details of what is not present in the table. Since there is no recors for the timestamp corresponding to the hour 8 (from your example) the SQL does not return any records.

Answer 3

I've finaly found the answer. Maybe I'm insane, but this works.

SELECT HOUR, max(HOUR_STAT) as HOUR_STAT FROM (
    (
        SELECT HOUR(TIMESTAMP_X) as HOUR, count(*) as HOUR_STAT
        FROM cms_webstat
        WHERE date(TIMESTAMP_X) = date(now())
    )
    UNION (SELECT 0 as HOUR, 0)
    UNION (SELECT 1 as HOUR, 0)
    UNION (SELECT 2 as HOUR, 0)
    UNION (SELECT 3 as HOUR, 0)
    UNION (SELECT 4 as HOUR, 0)
    UNION (SELECT 5 as HOUR, 0)
    UNION (SELECT 6 as HOUR, 0)
    UNION (SELECT 7 as HOUR, 0)
    UNION (SELECT 8 as HOUR, 0)
    UNION (SELECT 9 as HOUR, 0)
    UNION (SELECT 10 as HOUR, 0)
    UNION (SELECT 11 as HOUR, 0)
    UNION (SELECT 12 as HOUR, 0)
    UNION (SELECT 13 as HOUR, 0)
    UNION (SELECT 14 as HOUR, 0)
    UNION (SELECT 15 as HOUR, 0)
    UNION (SELECT 16 as HOUR, 0)
    UNION (SELECT 17 as HOUR, 0)
    UNION (SELECT 18 as HOUR, 0)
    UNION (SELECT 19 as HOUR, 0)
    UNION (SELECT 20 as HOUR, 0)
    UNION (SELECT 21 as HOUR, 0)
    UNION (SELECT 22 as HOUR, 0)
    UNION (SELECT 23 as HOUR, 0)
)
AS `combined_table`
GROUP BY HOUR
ORDER BY HOUR DESC

One MySQL query as desired.