TRIGGERS-Updating a value in a table not updating Oracle SQL - sql

Question

Table description:
two tables
Product (Prodid, Prodesc, Price, Stock,Reord)
Sales (Salesid, Proid, qty)
Question:
Create a Trigger which reduces the stock of Product that is been inserted in sales and update the stock when purchase is made.
I made a sample trigger only for purchase but my product's stock not updating.
This is my code.
SQL> create or replace trigger updat after
2 insert or update on product for each row
3 declare
4 temp number;
5 temp1 number;
6 temp2 varchar2(5);
7 begin
8 select qty into temp from purchase;
9 select proid into temp2 from purchase;
10 select stock into temp1 from product;
11 temp1:=temp1+temp;
12 update product set stock=temp1 where prodid=temp2;
13 end;
14 /
I am weak in Triggers. Help.

Answer 1

This trigger is a bit of a mess so it's hard to know quite where to start. I'll make a number of guesses and assumptions-- I'll try to call them out but I'll probably miss one or two.
First, the trigger likely needs to be defined on the sales table. You want to modify the product table in response to a change to the sales table. Second, the requirement only appears to require an after insert trigger, it doesn't appear that you need to modify the product table when a row in sales is updated.
CREATE OR REPLACE TRIGGER trigger_name
AFTER INSERT ON sales
FOR EACH ROW
Second, your variables are named exceptionally poorly. Calling things temp, temp1, and temp2 makes your code much harder to read and to write because you'll have no idea what any variables actually contain. That makes it much harder to read the code and see what's going on let alone notice any bugs. Always choose meaningful names.
Third, if you are going to write select into statements, you need to ensure that you are always selecting a single row. That almost certainly requires a WHERE clause, most likely using one or more columns from the :new pseudorecord to determine which row you are interested in. In this case, though, there doesn't appear to be a need to declare any local variables or to write any select statements. It sounds like you just want to
UPDATE product
SET stock = stock - :new.qty
WHERE prodID = :new.prodID
If you want to enhance your trigger to handle updates to sales in addition to inserts, you would want to compute the difference between the :old.qty and the :new.qty and subtract that from the stock of the product.

Answer 2

I am not sure why you are adding your qty with prodid. And the relation between the tables sales,product and purchase is not clear. From your description i think this would help.
create or replace trigger TRIG_STOCK
after insert or update on Sales
for each row
pragma autonomous_transaction;
begin
update product set stock=stock-:new.qty where prodid=:new.prodid;
update purchase set qty=:new.qty where prodid=:new.prodid;
commit;
end;