How can I know the number of remaining mondays, tuesdays, wednesdays, ..., and sundays in a month using SQL?

Question

I want to update the values of a row that has a date column and I want to generate the column values of RemainingMondays, RemainingTuesdays, etc. in that month. For example if I have 2019-03-20 (March 20th, 2019) I need to update that row with the next column values. RemainingMondays -> 2, RemainingTuesdays-> 1, RemainingWednesdays -> 1, RemainingThursdays -> 2, RemainingFridays -> 2, RemainingSaturdays -> 2, RemainingSundays -> 2, (as you can see) I don't want to count the weekday in that day (It means that in the last day of the month all the column values will have a value of 0). How to do this in SQL?

Answer 1

I tested this solution with MySQL, but it should works in any SQL engine.

Test data:

CREATE TABLE test_date (
  date date NOT NULL
);

INSERT INTO test_date VALUES
('2019-03-01'),
('2019-03-20'),
('2019-03-21'),
('2019-03-22'),
('2019-03-23'),
('2019-03-24'),
('2019-03-25'),
('2019-03-26');

The query to get your data:

SELECT 
    td2.date,
    /* 
    To calculate remaining of a weekday, you can then use this formula:

    if desired_week_day_number > week_day_number
       and desired_week_day_number <= (week_day_number + remaining_days_after_weeks):
        remaining_weeks + 1
    elseif desired_week_day_number <= (week_day_number + remaining_days_after_weeks - 7):
        remaining_weeks + 1
    else remaining_weeks
    */
    CASE WHEN (1 > td2.week_day_number AND 1 <= (td2.week_day_number + td2.remaining_days_after_weeks))
              OR 1 <= (td2.week_day_number + td2.remaining_days_after_weeks - 7)
         THEN td2.remaining_weeks + 1
         ELSE td2.remaining_weeks
    END remaining_sundays,
    CASE WHEN (2 > td2.week_day_number AND 2 <= (td2.week_day_number + td2.remaining_days_after_weeks))
              OR 2 <= (td2.week_day_number + td2.remaining_days_after_weeks - 7)
         THEN td2.remaining_weeks + 1
         ELSE td2.remaining_weeks
    END remaining_mondays,
    CASE WHEN (3 > td2.week_day_number AND 3 <= (td2.week_day_number + td2.remaining_days_after_weeks))
              OR 3 <= (td2.week_day_number + td2.remaining_days_after_weeks - 7)
         THEN td2.remaining_weeks + 1
         ELSE td2.remaining_weeks
    END remaining_tuesdays,
    CASE WHEN (4 > td2.week_day_number AND 4 <= (td2.week_day_number + td2.remaining_days_after_weeks))
              OR 4 <= (td2.week_day_number + td2.remaining_days_after_weeks - 7)
         THEN td2.remaining_weeks + 1
         ELSE td2.remaining_weeks
    END remaining_wednesdays,
    CASE WHEN (5 > td2.week_day_number AND 5 <= (td2.week_day_number + td2.remaining_days_after_weeks))
              OR 5 <= (td2.week_day_number + td2.remaining_days_after_weeks - 7)
         THEN td2.remaining_weeks + 1
         ELSE td2.remaining_weeks
    END remaining_thursdays, 
    CASE WHEN (6 > td2.week_day_number AND 6 <= (td2.week_day_number + td2.remaining_days_after_weeks))
              OR 6 <= (td2.week_day_number + td2.remaining_days_after_weeks - 7)
         THEN td2.remaining_weeks + 1
         ELSE td2.remaining_weeks
    END remaining_fridays,
    CASE WHEN (7 > td2.week_day_number AND 7 <= (td2.week_day_number + td2.remaining_days_after_weeks))
              OR 7 <= (td2.week_day_number + td2.remaining_days_after_weeks - 7)
         THEN td2.remaining_weeks + 1
         ELSE td2.remaining_weeks
    END remaining_saturdays,
    td2.*
FROM (
    SELECT
        td.date,
        day(td.date) day_number,
        dayofweek(td.date) week_day_number,
        dayname(td.date) week_day_name,
        day(last_day(td.date)) nb_month_days,
        day(last_day(td.date)) - day(td.date) remaining_days,
        floor((day(last_day(td.date)) - day(td.date)) / 7) remaining_weeks,
        day(last_day(td.date)) - day(td.date) -
            floor((day(last_day(td.date)) - day(td.date)) / 7) * 7 remaining_days_after_weeks
    FROM test_date td
) AS td2
ORDER BY td2.date;

You can plug the exact same thing in an update statement accordingly.

It can of course be made way shorter/simpler if you put the weekdays in a sub-query / sub-table, avoiding to repeat the formula for each day of the week, but I let this exercice to you for now. ;-) The hard of the work was mainly to figure out how to calculate the remaining of a given weekday.