-- create tables, items holds item number, name and amount sold.
-- sales holds sales transactions 
CREATE TABLE items (itemno int, itemname char(20), sold decimal(6,2));
CREATE TABLE sales (itemno int, price decimal(6, 2));

--create the trigger function
CREATE OR REPLACE FUNCTION add_sales() RETURNS TRIGGER AS $as$
BEGIN
  UPDATE items
  SET sold = sold + NEW.price
  WHERE items.itemno = NEW.itemno;
  RETURN NEW;
END $as$ LANGUAGE 'plpgsql';

--bind the trigger to sales
CREATE TRIGGER items_trigger AFTER INSERT ON sales
   FOR EACH ROW
   EXECUTE PROCEDURE add_sales();

--insert and view data
insert  into items(itemno, itemname, sold)
 values (5, 'book', 0.0),
        (6, 'bell', 0.0);
select  * from items;
select  * from sales;

--insert and view data
insert  into sales(itemno, price)
 values (5, 6.50),
        (6, 7.25);
insert  into sales(itemno, price)
 values (6, 9.25);
select  * from items;
select  * from sales;