Question

Consider the following “messages” table:

_date        message
-------------------------
2012-02-22   hello
2012-02-22   another msg!
2012-03-05   foobar
2012-03-22   testing
2012-03-22   goodbye
2012-03-22   test test

I want to count the number of messages per day, but also want to include all the days on which there are no messages. Say I want to count all messages between 2012-02-01 and 2012-03-29, then the query result should be:

_date        messsages
----------------------
2012-02-01   0
2012-02-02   0
...
2012-02-22   2
2012-02-23   0
...
2012-03-01   0
2012-03-02   0
2012-03-03   0
2012-03-04   0
2012-03-05   1
...
2012-03-22   3
...
2012-03-29   0

Is this possible within MySQL?

Answer 1

Something like this should do it –

SELECT date_range.`date`, COUNT(messages._date)
FROM (
    SELECT '2012-02-01' + INTERVAL (id - 1) DAY AS `date`
    FROM dummy
    WHERE id BETWEEN 1 AND 58
    ORDER BY id ASC
) AS date_range
LEFT JOIN messages
    ON date_range.`date` = messages._date
GROUP BY date_range.`date` ASC

For this example dummy is any table with an integer based id field with a contiguous set covering the required date range. In this case you would need ids from 1 to 58.