a postgreql example function for summarizing data

quarterly_summary_func.sql

CREATE FUNCTION quarterly_summary_func(start_date date DEFAULT CURRENT_TIMESTAMP)
RETURNS TABLE (staff_name text, staff_bonus int, quarter tsrange)
As $$
DECLARE
  employee RECORD;
  total_bonus int;
  sales_total int;
  end_date date := start_date + interval '3 months';
BEGIN
  FOR employee IN SELECT staff_id FROM staff LOOP
    EXECUTE 'SELECT sum(staff_bonus), sum(sales_price) FROM sales WHERE staff_id = $1 
    AND created_at >= $2 AND created_at < $3'
       INTO total_bonus, sales_total
       USING employee.staff_id, start_date, end_date;
    RAISE NOTICE 'total bonus is % and total sales is %', total_bonus, sales_total;
   EXECUTE 'INSERT INTO sales_summary (staff_id, bonus, total_sales, period) VALUES
              ($1, $2, $3, tsrange($4, $5))'
       USING employee.staff_id, total_bonus, sales_total, start_date, end_date;
  END LOOP;
  DELETE FROM sales WHERE created_at >= start_date
         AND created_at < end_date;
  RETURN QUERY SELECT name, bonus, period FROM sales_summary
               LEFT JOIN staff on sales_summary.staff_id = staff.staff_id;
 RETURN;
END;
$$
LANGUAGE plpgsql;