Question

I currently have the following SQL statement

MySQL Query:

SELECT
    c.day,
    COUNT(*)
FROM
    calendar c
JOIN
    visitors d
ON  DAYOFMONTH(d.created) = c.day
WHERE
    c.day BETWEEN DAYOFMONTH('2012-10-01') AND DAYOFMONTH('2012-10-31')
AND
    site_id = 16
GROUP BY
    DAYOFMONTH(d.created)
ORDER BY
    DAYOFMONTH(d.created)

My Tables:

Calendar

id  | day
---------
1   | 1
2   | 2
3   | 3
...
31  | 31

Visitors

id  | site_id | created
-----------------------------------
1   | 16      | 2012-10-18 11:14:39
2   | 16      | 2012-10-18 11:15:17
3   | 11      | 2012-10-18 11:49:14
4   | 11      | 2012-10-18 11:49:43
5   | 16      | 2012-10-19 11:54:37
6   | 1       | 2012-10-19 05:56:31
7   | 2       | 2012-10-19 05:57:56

I have created the table, calendar as prescribed in this answer but I seem to still get the same information. I am only getting the dates where I have data.

day    |   COUNT(*)
---------------------
18     |   2
19     |   1

I need to also retrieve 0 on the dates that have no data.

UPDATE:

I tried this:
SELECT *
FROM calendar c
LEFT JOIN visitors d
ON  DAYOFMONTH(d.created) = c.day

and

SELECT *
FROM calendar c
LEFT JOIN visitors d
ON  DAYOFMONTH(d.created) = c.day
WHERE  site_id = 16

I can confirm that the site_id = 16 is certainly the one killing the results.

Answer 1

use LEFT JOIN instead of INNER JOIN

SELECT ...
FROM   calendar c 
       LEFT JOIN  visitors d
         ON DAYOFMONTH(d.created) = c.day
WHERE...

INNER JOIN retrieves only rows which has atleast one match on the other table while LEFT JOIN retrieves all rows define on the lefthand side table whether it has a match or none on the other table(s).

UPDATE 1

SELECT  c.day,
        COUNT(*)
FROM    calendar c
        LEFT JOIN
        (
            SELECT *
            FROM   visitors
            WHERE  site_id = 16
        ) d ON  DAYOFMONTH(d.created) = c.day
WHERE c.day BETWEEN DAYOFMONTH('2012-10-01') AND DAYOFMONTH('2012-10-31')
GROUP BY DAYOFMONTH(c.day)
ORDER BY DAYOFMONTH(c.day)

**UPDATE by Thorpe Obazee

SELECT  c.day,
        COUNT(site_id)
FROM    calendar c
        LEFT JOIN
        (
            SELECT *
            FROM   visitors
            WHERE  site_id = 16
        ) d ON  DAYOFMONTH(d.created) = c.day
WHERE c.day BETWEEN DAYOFMONTH('2012-10-01') AND DAYOFMONTH('2012-10-31')
GROUP BY c.day
ORDER BY c.day

We cannot use COUNT(*) since it will return 1 every day. We also should not use DAYOFMONTH on c.day in the GROUP BY and ORDER BY since it is already what we need.