Good job! Of course, you can combine conditions in an IF statement with boolean operators: AND, OR, and NOT. You can also use all comparison operators you know from SQL: <, >, <=, >=, =, etc.

The number of personal days for most employees is 20. However, employees from IT departments (department IDs: 2, 3, and 4) hired before 2016 get 30 personal days:

CREATE FUNCTION get_personal_days(emp_id integer)
RETURNS integer AS $$
DECLARE
  start_year integer;
  dep_id integer;
BEGIN
  start_year := (SELECT EXTRACT(year FROM start_date) FROM person WHERE person_id = emp_id);
  dep_id := (SELECT department_id FROM person WHERE person_id = emp_id);
  IF dep_id IN (2, 3, 4) AND start_year < 2016 THEN
    RETURN 30;
  ELSE
    RETURN 20;
  END IF;
END;
$$ LANGUAGE plpgsql;