SQL: Trigger Syntax - sql

Question

I am doing some coursework where I am supposed to create a trigger that checks the stock level before an order is created, to see if there is enough product in stock to meet the order. Problem is the lecturer has not properly taught us the syntax of creating triggers, my issue being I dont know how to tell the trigger which row/collumn in the stocktable I am checking against. do I need to join the tables?
This is the code I have so far (I am just winging it here, it doesnt really work, but I'm putting it here to give you an idea of what it is I want to acchieve):
CREATE OR REPLACE TRIGGER stock_check
BEFORE INSERT OR UPDATE OF product_quantity ON orderline
FOR EACH ROW
DECLARE
Newtotal number;
insufficient_stock exception;
BEGIN
IF(:NEW.product_quantity > product_stock.stock_quantity) then
raise insufficient_stock;
Else
:new.product_stock.stock_quantity - product_quantity = newtotal
Update stock_quantity
Set product_quantity = newtotal
END IF;
exception
when insufficient_stock then
raise_application_error(-20604,'There is not enoguh stock available');
END;

Answer 1

I guess you'll need something like the following:
CREATE OR REPLACE TRIGGER stock_check
BEFORE INSERT ON orderline
FOR EACH ROW
DECLARE
oldtotal NUMBER
newtotal NUMBER;
insufficient_stock EXCEPTION;
BEGIN
SELECT stock_quantity INTO old_total
FROM product_stock
WHERE product_id = :new.product_id:
IF(:new.product_quantity > oldtotal) then
RAISE insufficient_stock;
ELSE
newtotal := oldtotal - :new.product_quantity;
UPDATE stock_quantity
SET product_quantity = newtotal
WHERE product_id = :new.product_id;
END IF;
EXCEPTION
WHEN insufficient_stock THEN
RAISE_APPLICATION_ERROR(-20604,'There is not enoguh stock available');
END;
You still have to add a column for some sort of product_id. And I didn't handle the UPDATE case...

Answer 2

for example:
SQL> create table product_stock (prod_id number, stock_quantity number);
Table created.
SQL> create table orderline (prod_id number, product_quantity number);
Table created.
SQL> insert into product_stock values (1, 1002);
1 row created.
SQL>
SQL> CREATE OR REPLACE TRIGGER stock_check
2 BEFORE INSERT OR UPDATE OF product_quantity ON orderline
3 FOR EACH ROW
4 DECLARE
5 v_stock_quantity product_stock.prod_id%type;
6 v_Newtotal number;
7 insufficient_stock exception;
8 BEGIN
9 if updating
10 then
11 v_Newtotal := :new.product_quantity - :old.product_quantity;
12 else
13 v_Newtotal := :new.product_quantity;
14 end if;
15
16 Update product_stock
17 Set stock_quantity = stock_quantity - v_Newtotal
18 where prod_id = :new.prod_id
19 returning stock_quantity into v_stock_quantity;
20
21 IF (v_stock_quantity < 0)
22 then
23 raise insufficient_stock;
24 END IF;
25 exception
26 when insufficient_stock then
27 raise_application_error(-20604,'There is not enoguh stock available');
28 END;
29 /
Trigger created.
SQL> show errors
No errors.
SQL> select * from product_stock;
PROD_ID STOCK_QUANTITY
---------- --------------
1 1002
SQL> insert into orderline values (1, 1000);
1 row created.
SQL> select * from product_stock;
PROD_ID STOCK_QUANTITY
---------- --------------
1 2
SQL> insert into orderline values (1, 3);
insert into orderline values (1, 3)
*
ERROR at line 1:
ORA-20604: There is not enough stock available
ORA-06512: at "DTD_TRADE.STOCK_CHECK", line 17
ORA-04088: error during execution of trigger 'DTD_TRADE.STOCK_CHECK'
SQL> select * from product_stock;
PROD_ID STOCK_QUANTITY
---------- --------------
1 2
update done first to create a lock on purpose (as if two sessions did this call at once, you want the second to block to avoid stock going under 0).