PostgreSQL - Generate series using subqueries - sql

Question

Using PostgreSQL, I need to accomplish the following scenario. I have a table called routine, where I store start_date and end_date columns. I have another table called exercises, where I store all the data related with each exercise and finally, I have a table called routine_exercise where I create the relationship between the routine and the exercise. Each routine can have seven days (one day indicates the day of the week, e.g: 1 means Monday, etc) of exercises and each day can have one or more exercise. For example:
Exercise Table
Exercise ID
Name
1
Exercise 1
2
Exercise 2
3
Exercise 3
Routine Table
Routine ID
Name
1
Routine 1
2
Routine 2
3
Routine 3
Routine_Exercise Table
Exercise ID
Routine ID
Day
1
1
1
2
1
1
3
1
1
1
1
2
2
1
3
3
1
4
The thing that I'm trying to do is generate a series from start_date to end_date (e.g 03-25-2020 to 05-25-2020, two months) and assign to each date the number of day it supposed to work.
For example, using the data in the Routine_Exercise Table the user should only workout days: 1,2,3,4, so I would like to attach that number to each date. For example, something like this:
Expected Result
Date
Number
03-25-2020
1
03-26-2020
2
03-27-2020
3
03-28-2020
4
03-29-2020
null
03-30-2020
null
03-31-2020
null
04-01-2020
1
04-02-2020
2
04-03-2020
3
04-04-2020
4
04-05-2020
null
Any suggestions or different ideas on how to implement this? Another solution that doesn't require series?
Thanks in advance!

Answer 1

You can generate the dates between start and end input dates using generate_series and then do left join with your routine_exercise table as follows:
SELECT t.d, re.day
FROM generate_series(timestamp '2020-03-25', timestamp '2020-05-25',
interval '1 day') AS t(d)
left join (select distinct day from Routine_Exercise re WHERE ROUTINE_ID = 1) re
on mod(extract(day from (t.d -timestamp '2020-03-25')), 7) + 1 = re.day;