-- available online in file 'examp1'
DECLARE
   qty_on_hand  NUMBER(5);
BEGIN
   SELECT quantity INTO qty_on_hand FROM inventory
      WHERE product = 'TENNIS RACKET'
      FOR UPDATE OF quantity;
   IF qty_on_hand > 0 THEN  -- check quantity
      UPDATE inventory SET quantity = quantity - 1
         WHERE product = 'TENNIS RACKET';
      INSERT INTO purchase_record
         VALUES ('Tennis racket purchased', SYSDATE);
   ELSE
      INSERT INTO purchase_record
         VALUES ('Out of tennis rackets', SYSDATE);
   END IF;
   COMMIT;
END;

-- From https://docs.oracle.com/cd/B10500_01/appdev.920/a96624/01_oview.htm#7106