Trying to get a Trigger and Alert to work

Question

So im trying to get a trigger to work with an alert and the Alert seems to be right and the trigger complies which seems right to me, however the instruction that I have in my book does not produce the same output that I get from my Update.
Here is the deal. I am to log into sql * with a default account as well as login as "SYSTEM"
the trigger should invoke the Alert and output a message to re-order some more product and the status should = 0 since there is no wait time. However I don't get a "Message" from the Alert and the status = 1 which indicates timeout. So if you can take a look at my code and let me know what I did wrong or how to "Connect" the two that would be great.
Trigger I created.
CREATE OR REPLACE TRIGGER order_replace_trg
AFtER UPDATE OF stock on bb_product
FOR EACH ROW
WHEN (OLD.stock = 24 AND NEW.stock = -2)
DECLARE
stock NUMBER(5,1);
idproduct NUMBER(2);
lv_msg_txt VARCHAR2(25);
lv_status_num NUMBER(1);
reorder NUMBER(3);
BEGIN
IF stock <> 24 AND reorder = 25 THEN
lv_msg_txt := 'Product 4 Reorder Time!';
DBMS_OUTPUT.PUT_LINE(lv_msg_txt);
ELSE
lv_status_num := 0;
DBMS_OUTPUT.PUT_LINE(lv_status_num);
END IF;
END;
The Alert:
BEGIN
DBMS_ALERT.REGISTER('reorder');
END;
DECLARE
lv_msg_txt VARCHAR2(25);
lv_status_num NUMBER(1);
BEGIN
DBMS_ALERT.WAITONE('reorder', lv_msg_txt, lv_status_num, 120);
DBMS_OUTPUT.PUT_LINE('Alert: ' ||lv_msg_txt);
DBMS_OUTPUT.PUT_LINE('Status: ' ||lv_status_num);
END;
Here is the block I need to run to test the trigger and alert.
UPDATE bb_product
SET stock = stock -2
WHERE idproduct = 4;
COMMIT;
The message I should get is:
Alert: Product 4 Reorder Time!
Status: 0
PL/SQL procedure successfully completed.
This is what I get.
SQL> /
Alert:
Status: 1
PL/SQL procedure successfully completed.
Thanks for your help!
Mac

Answer 1

Right. Register says "I'm interested in getting alerted to some particular event", Waitone says "I'm waiting until some event happens". Signal is the key thing that indicates that a particular event happened.
As for your trigger, a couple of issues
- I don't know why you're calling DBMS_OUTPUT. I'm guessing that you probably want to send a message along with your alert that the receiver gets and displays, not that you want to print a message to the window from inside the trigger.
- You're using the local variables stock and reorder in your IF statement but you never initialize them. I'm guessing that you would want to eliminate those local variables and just use :new.stock and :new.reorder (assuming that REORDER is a column in the table).
- Your WHEN clause doesn't seem to make sense. It's telling the trigger to fire only if you update stock from 24 to -2, which doesn't make sense. I'm not sure you would even need a WHEN clause here.
Justin