MySQL to get the count of rows that fall on a date for each day of a month

Question

I have a table that contains a list of community events with columns for the days the event starts and ends. If the end date is 0 then the event occurs only on the start da

Answer 1

Unfortunately, MySQL lacks a way to generate a rowset of given number of rows.

You can create a helper table:

CREATE TABLE t_day (day INT NOT NULL PRIMARY KEY)

INSERT
INTO    t_day (day)
VALUES  (1),
        (2),
        …,
        (31)

and use it in a JOIN:

SELECT  day, COUNT(*)
FROM    t_day
JOIN    p_community e
ON      day BETWEEN DATE(e.start) AND IF(DATE(e.end), DATE(e.end), DATE(e.start))
GROUP BY
        day

Or you may use an ugly subquery:

SELECT  day, COUNT(*)
FROM    (
        SELECT  1 AS day
        UNION ALL
        SELECT  2 AS day
        …
        UNION ALL
        SELECT  31 AS day
        ) t_day
JOIN    p_community e
ON      day BETWEEN DATE(e.start) AND IF(DATE(e.end), DATE(e.end), DATE(e.start))
GROUP BY
        day