MySQL function IFNULL not working with GROUP BY - mysql

Question

I've got a standard table with the list of users and I've got a column lastactivity with UNIX Timestamp (which shows when they have logged in) and column timestamp with UNIX Timestamp that shows when they have registered.
I've build a SQL query that shows how many users were active within 24 hours (86400 seconds) from now and grouped results by weeks so the counter counts how many users have registered each week:
SELECT
IFNULL(COUNT(*),0) as `counter`,
(WEEK(`timestamp`)) as `week`
FROM
`clients`
WHERE
(CAST(UNIX_TIMESTAMP() as signed) - CAST(`lastactivity` as signed)) <= 86400
GROUP BY
WEEK(`timestamp`);
The issue is that function IFNULL(COUNT(*),0) is not working as I intended. This SQL query won't display the week if there is NULL / 0 on the counter even with IFNULL() MySQL function. That is probably because of how GROUP BY works. So for example I will get this kind of result:
counter | week
2 | 11
1 | 13
9 | 14
6 | 17
But I would like to show each week like this:
counter | week
2 | 11
0 | 12
1 | 13
9 | 14
0 | 15
0 | 16
6 | 17
Anyone have idea how can I fix this issue?
Gordon is trying to help me by getting LEFT JOIN query but I still got the same results, maybe I am doing something wrong here:
SELECT
COUNT(a.id) as `counter`,
(WEEK(b.timestamp)) as `week`
FROM
`users` a
LEFT JOIN
`users` b
ON
a.id = b.id
WHERE
(CAST(UNIX_TIMESTAMP() as signed) - CAST(a.lastactivity as signed)) <= 86400
GROUP BY
WEEK(b.timestamp);

Answer 1

The problem is that you don't understand how the query works. IFNULL() (or the standard version COALESCE() converts a column value that is NULL to some other value. However, COUNT() never returns NULL. So, leave it out:
SELECT COUNT(*) as `counter`, WEEK(`timestamp`) as `week`
FROM `clients`
WHERE (CAST(UNIX_TIMESTAMP() as signed) - CAST(`lastactivity` as signed)) <= 86400
GROUP BY WEEK(`timestamp`);
Your problem is missing rows, not NULL values. You would have to solve this with a LEFT JOIN.
EDIT:
You need a left join to include all the weeks:
SELECT COUNT(c.timestamp) as `counter`, wk as `week`
FROM (SELECT 11 as wk UNION ALL
SELECT 12 UNION ALL
SELECT 13 UNION ALL
SELECT 14 UNION ALL
SELECT 15 UNION ALL
SELECT 16 UNION ALL
SELECT 17
) w LEFT JOIN
`clients` c
ON WEEK(c.`timestamp`) = w.wk
WHERE (CAST(UNIX_TIMESTAMP() as signed) - CAST(`lastactivity` as signed)) <= 86400
GROUP BY WEEK(`timestamp`);