PL/SQL: Run package after checking all rows in trigger - plsql

Question

I have an inventory table with expected quantities and actually received quantities. Let's say inv.q_ex and inv.q_rd.
The INSERT to the table has a positive value in q_ex and a zero in q_rd because it hasn't arrived yet. I'd like to run a package when I detect that the q_rd value changes from 0 to something else, indicating it's been received and stored.
Making a trigger to detect after update and checking each row is easy, but I'm not sure how to ensure it only runs once.
The skeleton is:
CREATE OR REPLACE TRIGGER example
AFTER UPDATE ON inv
FOR EACH ROW
BEGIN
IF :OLD.q_rd = 0 AND :NEW.q_rd > 0 THEN
pkg.proc();
END IF;
END;
/
The problem I see is I only want it to run one time. I just need to identify when it needs to be executed. Ideally, on the first row where my condition is met, I would exit the loop (seems like a waste to keep checking when I already know I need to execute) and call my procedure.
I couldn't find a way to "exit" the for each and treat it as a normal AFTER UPDATE, so then I tried using both BEFORE UPDATE and AFTER UPDATE. The BEFORE portion would check each row and update a boolean. The AFTER portion would wait for that to happen and if it was true, call the procedure.
CREATE OR REPLACE TRIGGER example
BEFORE UPDATE ON inv
FOR EACH ROW
DECLARE
shouldExecute BOOLEAN;
BEGIN
IF :OLD.q_rd = 0 AND :NEW.q_rd > 0 THEN
shouldExecute := TRUE;
END IF;
END;
AFTER UPDATE ON inv
BEGIN
IF shouldExecute THEN
pkg.proc();
END IF;
END;
/
I suspect this wouldn't work anyway because, according to the syntax, it redeclares the boolean variable on each row. I thought that maybe I could make it "global" but regardless, turns out I can't add both BEFORE and AFTER to the same trigger for some reason (unless I didn't research enough), so I broke it out into two triggers. The problem now is I can't share that boolean between the two triggers. Can I share the value, or am I going about this all wrong?

Answer 1

There's a lot of little questions here so I'll try to answer them all :)
Regarding "FOR EACH ROW", Oracle triggers support two different triggering methods, STATEMENT or ROW. If you include the "FOR EACH ROW" in the definition, you'll get a row trigger, which is fired once per row affected by the query, which is what you seem to want here. Statement level triggers get fired only once for each query. An advantage to using row triggers is that you can use the :OLD and :NEW metavariables which refer to the previous and current row values.
As you've discovered, you can't add BEFORE and AFTER to the same trigger - you'll need to break them out into two triggers.
Unfortunately there isn't a simple way of sharing the boolean variable between the two triggers. The easiest way is probably to create a package with a public variable, which you can set in the BEFORE trigger, and check in the AFTER trigger.
The package would look something like this:
CREATE OR REPLACE PACKAGE PCKG_DEMO
AS
shouldExecute BOOLEAN;
END;
/
Then your BEFORE UPDATE trigger:
CREATE OR REPLACE TRIGGER beforeexample
BEFORE UPDATE ON inv
FOR EACH ROW
BEGIN
IF :OLD.q_rd = 0 AND :NEW.q_rd > 0 THEN
PCKG_DEMO.shouldExecute := TRUE;
END IF;
END;
/
And your AFTER UPDATE trigger:
CREATE OR REPLACE TRIGGER afterexample
AFTER UPDATE ON inv
FOR EACH ROW
BEGIN
IF PCKG_DEMO.shouldExecute THEN
pkg.proc();
END IF;
END;
/
Those are a bit pseudocode-y - I don't have access to an Oracle database right now. You can read more about triggers here. Hope this helps!

Answer 2

You need to do more research. The link above takes you to trigger discussion for Oracle 9. I hope you are not actually using that version; support ended in 2007. Since version 11g Oracle has provided "Compound Triggers" where you can fire the same trigger both Before and After for both statement and row level. Compound triggers do allow sharing variables between the different invocations.