PL/SQL Conditional statement using two tables within trigger - sql

Question

I'm working on a simple trigger that updates the Quantity on Hand of a product when adding a new invoice line.
I got that working fine; but I starting thinking that in a practical application it would be prudent to verify that the QOH is sufficient before allowing the update. I'm sure I could write a separate trigger for that, but I'd like to see if it's possible to join two tables for a conditional statement within a trigger.
This is as far as I've gotten; adding a SELECT statement anywhere in there causes all hell to break loose, so I'm a little stumped how I can declare the PRODUCT.P_QOH before calling the conditional.
CREATE OR REPLACE TRIGGER trg_prod_QOH_on_line_add
BEFORE INSERT ON LINE
FOR EACH ROW
BEGIN
IF :NEW.LINE_UNITS > PRODUCT.P_QOH THEN
RAISE_APPLICATION_ERROR(-20202, 'Insufficient quantity on hand');
ELSE
UPDATE PRODUCT
SET P_QOH = P+QOH - :NEW.LINE_UNITS;
WHERE PRODUC.P_CODE = :NEW.P_CODE;
END IF;
END;
/
This isn't a major problem for me, as I said there is probably a different way; I'm just starting to learn this stuff and would like to see what's possible. Thanks for your help.

Answer 1

You get into dangerous territory trying to enforce rules like this via triggers. The solution you are asking for is:
create or replace trigger trg_prod_qoh_on_line_add
before insert on line
for each row
declare
v_qoh product.p_qoh%type;
begin
select p_qoh
into v_qoh
from product
where product.p_code = :new.p_code;
if :new.line_units > v_qoh then
raise_application_error(-20202, 'Insufficient quantity on hand');
else
update product
set p_qoh = p_qoh - :new.line_units
where product.p_code = :new.p_code;
end if;
end;
However this is not a safe solution in a system with more than one concurrent user. Suppose product 'X' has p_qoh=10 and then 2 users do this:
user1> insert into line (p_code, line_units) values ('X', 7);
user2> insert into line (p_code, line_units) values ('X', 8);
user1> commit;
user2> commit;
Both sessions will see that 'X' has p_qoh = 10 so both will succeed and product.p_qoh will end up as -5. All is corrupt!
The safe solution would be to create a check constraint on product:
alter table product add constraint prod_qoh_chk check (p_qoh >= 0);
Now your trigger can be simply:
create or replace trigger trg_prod_qoh_on_line_add
before insert on line
for each row
begin
update product
set p_qoh = p+qoh - :new.line_units;
where produc.p_code = :new.p_code;
end;
This would raise a less friendly error message like:
ORA-02290: check constraint (MYSCHEMA.PROD_QOH_CHECK) violated
You can trap this in your trigger and give the message you want:
create or replace trigger trg_prod_qoh_on_line_add
before insert on line
for each row
begin
update product
set p_qoh = p+qoh - :new.line_units;
where produc.p_code = :new.p_code;
exception
when others then
if sqlerrm like 'ORA-02291:%(MYSCHEMA.PROD_QOH_CHECK)%' then
raise_application_error(-20202,'Insufficient quantity on hand');
else
raise;
end if;
end;
Now if we re-run the 2 user scenario above:
user1> insert into line (p_code, line_units) values ('X', 7);
user2> insert into line (p_code, line_units) values ('X', 8);
user1> commit;
At this point user2's insert fails with the error message:
ORA-20202: Insufficient quantity on hand