Exception Handling SQL - oracle

Question

I created a user defined function that calculates the quantity in stock for a product
CREATE OR REPLACE FUNCTION function_quantityInStock(
oldProductQuantity IN INTEGER,
orderedQuan IN INTEGER)
RETURN INTEGER
IS
v_newQuantity INTEGER;
v_oldQuantity INTEGER;
v_orderedQuan INTEGER;
BEGIN
v_newquantity := oldProductQuantity - orderedQuan;
RETURN v_newquantity;
EXCEPTION
WHEN OTHERS THEN
dbms_output.put_line('Please check your data.');
END function_quantityInstock;
I then create a trigger that updates the table by calling the function
CREATE OR REPLACE TRIGGER TRIGGER_QUANTITY AFTER INSERT ON ordered_product
FOR EACH ROW
DECLARE
v_oldQuantity INTEGER;
BEGIN
SELECT PRODUCT_QUANTITYINSTOCK INTO v_oldQuantity
FROM product
WHERE product_id = :NEW.product_id;
UPDATE PRODUCT
SET product_quantityinstock =
function_quantityINSTOCK(v_oldQuantity, :NEW.ORDERED_PRODUCTQUANTITY)
WHERE product_id = :NEW.product_id;
END;
I want to display a message when the user enters invalid data but my exception block doesnt do that.
I use the following anonyomus block to test the function:
SET SERVEROUTPUT ON;
DECLARE
v_productID ordered_product.product_id%TYPE:= &ProductID;
v_orderID ordered_product.order_id%TYPE:=&OrderID;
v_orderedQuan ordered_product.ordered_productQuantity%TYPE := &OrderedProductQuantity;
v_totalCost ordered_product.ordered_productTotalCost%TYPE := '&TotalCost';
BEGIN
INSERT INTO ordered_product VALUES
(v_orderID, v_productID, v_orderedQuan, v_totalCost);
dbms_output.put_line('A new record has been inserted.');
EXCEPTION
WHEN NO_DATA_FOUND THEN
dbms_output.put_line('Invalid data!');
WHEN VALUE_ERROR THEN
dbms_output.put_line('Error! Please check your values.'|| SQLERRM);
END;

Answer 1

You might be missing exec dbms_output.enable(10000) and therefore not be seeing the output.
You should throw the exception using
raise_application_error(-20000, 'Please check your data.');
Also it's bad practice to catch all exceptions using when others and then ignore them.

Answer 2

If I were you, I wouldn't rely on dbms_output.put_line to pass information around. Instead, rely on the standard exception handling, e.g. RAISE or RAISE_APPLICATION_ERROR. Also, given that you're trying to find the new quantity, I'd move the select on the product table into the function, something like:
CREATE OR REPLACE FUNCTION function_quantityinstock(p_product_id IN product.product_product_id p_orderedquan IN INTEGER)
RETURN INTEGER IS
v_newquantity INTEGER;
e_not_enough_stock EXCEPTION;
e_not_enough_stock_num INTEGER := -20001;
e_no_product_exists_num INTEGER := -20002;
PRAGMA EXCEPTION_INIT(e_not_enough_stock, -20001);
BEGIN
SELECT product_quantityinstock - orderedquan
INTO v_newquantity
FROM product
WHERE product_id = :new.product_id;
IF v_newquantity < 0
THEN
RAISE e_not_enough_stock;
END IF;
RETURN v_newquantity;
EXCEPTION
WHEN no_data_found THEN
raise_application_error(-e_no_product_exists_num,
'No product exists for product_id = ' || product_id);
-- no need to check for TOO_MANY_ROWS if product_id is the primary/unique key on the product table.
WHEN e_not_enough_stock THEN
raise_application_error(e_not_enough_stock_num,
'Not enough stock present to fulfil the order for product_id = ' ||
product_id);
WHEN OTHERS THEN
raise_application_error(SQLCODE,
'Unexpected error occurred whilst finding new quantity for product_id = ' ||
product_id || ': ' || SQLERRM);
END function_quantityinstock;
/
You'd need to change the function name though, to reflect the action that it's doing, since it's returning the new quantity in stock...