Counting number of rows between 2 dates and 1 where clause - mysql

Question

I've a mysql table with me
Now we want to do some calculations like this
count date wise for all courses enrolled
count where course id = 2 for date > start_date AND date < end_date
Expected output where we calculate all courses enrolled
Expected output where we calculate all courses enrolled where course id = 2
*
expected output where course_id = 2 AND date range is between 2022-11-15 to 2022-11-13
The query which I've right now
SELECT COUNT(*), DATE(registered_on)
FROM courses_enrolled
WHERE course_id = 1
GROUP BY DATE(registered_on), course_id
ORDER BY registered_on desc;

Answer 1

You need to use some kind of calendar table approach here:
SELECT d.dt AS date, COUNT(ce.id) AS cnt
FROM (
SELECT '2022-11-12' AS dt UNION ALL
SELECT '2022-11-13' UNION ALL
SELECT '2022-11-14' UNION ALL
SELECT '2022-11-15'
) d
LEFT JOIN courses_enrolled ce
ON DATE(ce.registered_on) = d.dt AND
ce.course_id = 2
GROUP BY d.dt
ORDER BY d.dt;
The calendar table ensures that all dates you want in the output appear. In practice, you may replace the subquery in d with a bona-fide table containing all dates of interest. The left join ensures that no dates are dropped which have no matching courses on that day.

Answer 2

If you are using MySQL 8 you can use a recursive CTE to create your date range.
For all enrolled courses for given date range -
WITH RECURSIVE calendar (date) AS (
SELECT '2022-11-13' # start date
UNION ALL
SELECT date + INTERVAL 1 DAY FROM calendar
WHERE date + INTERVAL 1 DAY <= '2022-11-15' # end date
)
SELECT COUNT(ce.id) count_all, c.date
FROM calendar c
LEFT JOIN courses_enrolled ce
ON ce.registered_on BETWEEN c.date AND (c.date + INTERVAL 1 DAY - INTERVAL 1 SECOND)
GROUP BY c.date
ORDER BY c.date DESC;
Note the use of BETWEEN start AND end of day in the join criteria. For a small dataset this offers negligible benefit but on a large dataset it would allow for use of an index on registered_on, which could offer significantly improved performance.
Or for just the selected course -
WITH RECURSIVE calendar (date) AS (
SELECT '2022-11-13' # start date
UNION ALL
SELECT date + INTERVAL 1 DAY FROM calendar
WHERE date + INTERVAL 1 DAY <= '2022-11-15' # end date
)
SELECT COUNT(ce.id) count_selected_course, c.date
FROM calendar c
LEFT JOIN courses_enrolled ce
ON ce.registered_on BETWEEN c.date AND (c.date + INTERVAL 1 DAY - INTERVAL 1 SECOND)
AND ce.course_id = 2
GROUP BY c.date
ORDER BY c.date DESC;
Or counting both at the same time -
WITH RECURSIVE calendar (date) AS (
SELECT '2022-11-13' # start date
UNION ALL
SELECT date + INTERVAL 1 DAY FROM calendar
WHERE date + INTERVAL 1 DAY <= '2022-11-15' # end date
)
SELECT COUNT(ce.id) count_all, COUNT(IF(ce.course_id = 2, ce.id, NULL)) count_selected_course, c.date
FROM calendar c
LEFT JOIN courses_enrolled ce
ON ce.registered_on BETWEEN c.date AND (c.date + INTERVAL 1 DAY - INTERVAL 1 SECOND)
GROUP BY c.date
ORDER BY c.date DESC;