How to add day gaps to mysql query without calendar table - mysql

Question

I would like to receive the sum of all requests of the last 10 days grouped by date per day.
If there was no request on a day, the corresponding date should appear with sumrequests = 0.
My current query (today is the date 2020-01-10):
SELECT
count( 0 ) AS sumrequests,
cast( requests.created_at AS date ) AS created
FROM
requests
WHERE
(
requests.created_at
BETWEEN ( curdate() - INTERVAL 10 DAY )
AND ( curdate() + INTERVAL 1 DAY ))
GROUP BY
cast(requests.created_at AS date)
I then receive the following list:
sumrequests | created
--------------------------
3 | 2020-01-05
100 | 2020-01-08
But it should give back:
sumrequests | created
--------------------------
0 | 2020-01-01
0 | 2020-01-02
0 | 2020-01-03
0 | 2020-01-04
3 | 2020-01-05
0 | 2020-01-06
0 | 2020-01-07
100 | 2020-01-08
0 | 2020-01-09
0 | 2020-01-10
How can I get this without an additional calendar table.
Thanks for help!

Answer 1

For just 10 days of data, you can simply enumerate the numbers; using this derived number table, you can generate the corresponding date range, left join it with the table and aggregate.
SELECT
COALESCE(count(r.created_at), 0) AS sumrequests,
CURDATE() - INTERVAL (n.i) DAY AS created
FROM (
select 0 i 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
) n
LEFT JOIN requests r
ON r.created_at >= CURDATE() - INTERVAL n.i DAY
AND r.created_at < CURDATE() - INTERVAL (n.i - 1) DAY
GROUP BY n.i
ORDER BY n.i DESC
Side notes:
generally you want to avoid applying functions in the join or filtering conditions, since it prevents the use of an index; I modified your filters to not use CAST()
Since we are left joining, we need to count something that is coming from the requests table, hence we use COUNT(r.created_at) instead of COUNT(0)