source: database/Advanced Database Developement/Triggers/Automatic update of product availability after an order.txt@ 06ebe74

CREATE OR REPLACE FUNCTION update_product_availability()
RETURNS TRIGGER
LANGUAGE plpgsql
AS $$
BEGIN
    -- When a product is newly added to an order,
    -- decrease the available quantity.
    IF TG_OP = 'INSERT' THEN

        UPDATE sells
        SET quantity = quantity - (
            SELECT o.quantity
            FROM "order" o
            WHERE o.order_num = NEW.order_num
        )
        WHERE code = NEW.code;

    -- If the product/order entry is modified,
    -- restore the old quantity and subtract the new quantity.
    ELSIF TG_OP = 'UPDATE' THEN

        UPDATE sells
        SET quantity = quantity
            + (
                SELECT o.quantity
                FROM "order" o
                WHERE o.order_num = OLD.order_num
            )
            - (
                SELECT o.quantity
                FROM "order" o
                WHERE o.order_num = NEW.order_num
            )
        WHERE code = NEW.code;

    END IF;

    RETURN NULL;
END;
$$;


CREATE TRIGGER trg_update_product_availability
AFTER INSERT OR UPDATE
ON includes
FOR EACH ROW
EXECUTE FUNCTION update_product_availability();