IF-THEN-ELSIF-ELSE

CREATE FUNCTION ret_date (crdate date) RETURNS text AS $$
BEGIN
   IF EXTRACT (MONTH FROM crdate)= 1
   THEN RETURN 'January';
   ELSIF EXTRACT (MONTH FROM crdate)= 2
   THEN RETURN 'February';
   ELSIF EXTRACT (MONTH FROM crdate)= 3
   THEN RETURN 'March';
   ELSIF EXTRACT (MONTH FROM crdate)= 4
   THEN RETURN 'April';
   ELSIF EXTRACT (MONTH FROM crdate)= 5
   THEN RETURN 'May';
   ELSIF EXTRACT (MONTH FROM crdate)= 6
   THEN RETURN 'June';
   ELSIF EXTRACT (MONTH FROM crdate)= 7
   THEN RETURN 'July';
   ELSIF EXTRACT (MONTH FROM crdate)= 8
   THEN RETURN 'August';
   ELSIF EXTRACT (MONTH FROM crdate)= 9
   THEN RETURN 'September';
   ELSIF EXTRACT (MONTH FROM crdate)= 10
   THEN RETURN 'October';
   ELSIF EXTRACT (MONTH FROM crdate)= 11
   THEN RETURN 'November';
   ELSIF EXTRACT (MONTH FROM crdate)= 12
   THEN RETURN 'December';
   END IF;
END;
$$
LANGUAGE plpgsql;

SELECT ret_date(CURRENT_DATE);
select ret_date('2017-04-05');