How to Dynamically Generate a CASE Statement in Postgres for Date-Based Set Assignment?

Splitting Given Strings

To split the given dates (p_dates) and sets (p_sets), we can utilize the string_split() function of Postgres. string_split() takes two arguments: the input string to be split and a delimiter.

SELECT string_split(p_dates, ',');
SELECT string_split(p_sets, ',');

This will return two arrays containing the split dates and sets.

Preparing Dynamic Case Statement

To prepare a dynamic case statement, we can loop through the split arrays and construct the statement using a CASE expression.

DECLARE
  p_dates TEXT := '2001-01-01to2001-01-05,2001-01-10to2001-01-15';
  p_sets TEXT := 's1,s2';
  CASE_STATEMENT TEXT := 'CASE ';
BEGIN
  FOR p_date IN SELECT * FROM string_split(p_dates, ',') LOOP
    CASE_STATEMENT := CASE_STATEMENT || 'WHEN given_dates BETWEEN ' || quote_literal(p_date) || ' ' || ...
  END LOOP;

  FOR p_set IN SELECT * FROM string_split(p_sets, ',') LOOP
    CASE_STATEMENT := CASE_STATEMENT || 'THEN ' || quote_literal(p_set) || ' '
  END LOOP;
  
  -- Complete Case Statement with ELSE clause to assign NULL if no match
  CASE_STATEMENT := CASE_STATEMENT || 'ELSE NULL' || ' END';
END;
SELECT CASE_STATEMENT;

The resulting CASE_STATEMENT will be a dynamic case expression that assigns appropriate set names based on the given date ranges.

CASE 
  WHEN given_dates BETWEEN '2001-01-01' AND '2001-01-05' THEN 's1' 
  WHEN given_dates BETWEEN '2001-01-10' AND '2001-01-15' THEN 's2' 
  ELSE NULL 
END

You can use this dynamic CASE_STATEMENT in your update query to assign set names as per the given date ranges and set names.

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