Is it possible to generate columns based on input columns in PostgreSQL

Question

I have a table like this in PostgreSQL with 3 initial columns called Project, Size, and StartDate (see below):

Project	Size	StartDate
Project1	88	2020-06-15
Project2	105	2020-03-01

I need to add 12 columns next to StartDate column and extend the initial table to 15 columns. All new columns represent months for given year (2020), and they will contain 0 and values calculated based on conditional logic derived from Size and StartDate columns. The condition is the following: if the StartDate belongs to a specific month, that month gets a value=Size and after that for each next month the Value=Value-50 until Value >0. Please check the expected outcome below:

Project	Size	StartDate	Jan	Feb	Mar	Apr	Mai	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Project1	88	2020-06-15	0	0	0	0	88	38	0	0	0	0	0	0
Project2	105	2020-03-01	0	0	105	55	5	0	0	0	0	0	0	0

Answer 1

This can be done with a lot of CASE expressions:

SELECT project,
       size,
       startdate,
       CASE WHEN mon < 2
            THEN greatest(size - 50 * (1 - mon), 0)
            ELSE 0
       END AS jan,
       CASE WHEN mon < 3
            THEN greatest(size - 50 * (2 - mon), 0)
            ELSE 0
       END AS feb,
       CASE WHEN mon < 4
            THEN greatest(size - 50 * (3 - mon), 0)
            ELSE 0
       END AS mar,
       CASE WHEN mon < 5
            THEN greatest(size - 50 * (4 - mon), 0)
            ELSE 0
       END AS apr,
       CASE WHEN mon < 6
            THEN greatest(size - 50 * (5 - mon), 0)
            ELSE 0
       END AS may,
       CASE WHEN mon < 7
            THEN greatest(size - 50 * (6 - mon), 0)
            ELSE 0
       END AS jun,
       CASE WHEN mon < 8
            THEN greatest(size - 50 * (7 - mon), 0)
            ELSE 0
       END AS jul,
       CASE WHEN mon < 9
            THEN greatest(size - 50 * (8 - mon), 0)
            ELSE 0
       END AS aug,
       CASE WHEN mon < 10
            THEN greatest(size - 50 * (9 - mon), 0)
            ELSE 0
       END AS sep,
       CASE WHEN mon < 11
            THEN greatest(size - 50 * (10 - mon), 0)
            ELSE 0
       END AS oct,
       CASE WHEN mon < 12
            THEN greatest(size - 50 * (11 - mon), 0)
            ELSE 0
       END AS nov,
       CASE WHEN mon < 13
            THEN greatest(size - 50 * (12 - mon), 0)
            ELSE 0
       END AS dec
FROM (SELECT project,
             size,
             startdate,
             extract(month FROM startdate) AS mon
      FROM mytable) AS q;