Unknown error during execution of trigger - database

Question

I've got an insert/update trigger set up which prevents an employee from existing in two tables at the same time. It works fine with catching illegal insertions/updates but i'm also getting another error report when testing the trigger with illegal insertions/updates.
Here's my code:
CREATE OR REPLACE TRIGGER check_foobar
BEFORE INSERT OR UPDATE OF VarX ON FOOBAR
FOR EACH ROW
DECLARE
counter NUMBER(38);
BEGIN
SELECT count(*)
INTO counter
FROM BARFOO
WHERE VarX = :NEW.VarX
GROUP BY VarX;
IF counter > 0 THEN
RAISE_APPLICATION_ERROR(-20001, 'This is an illegal insertion/update');
END IF;
EXCEPTION WHEN NO_DATA_FOUND THEN DBMS_OUTPUT.PUT_LINE('TEST');
END;
/
And the errors ORA-06512 and ORA-04088 i'm not sure of:
SQL> INSERT INTO DRIVER VALUES(2, 10345, 'AVAILABLE');
Error starting at line : 26 File #test.sql
In command -
INSERT INTO FOOBAR VALUES(2)
Error report -
ORA-20001: This is an illegal insertion/update
ORA-06512: at "HR.CHECK_FOOBAR", line 11
ORA-04088: error during execution of trigger 'HR.CHECK_FOOBAR'
When i add an exception handler for the select statement my trigger stops working properly and the illegal insertion isn't prevented. But the execution error is prevented.
UPDATE: I've added a group by and an exception to the trigger, so now the trigger still works with an exception handler but the errors ORA-06512 and ORA-04088 are still coming up with the illegal insertion/update.
Line 11 mentioned in the error is
GROUP BY VarX;
Any advice would be much appreciated.

Answer 1

There is no problem here, it is working as expected.
Error starting at line : 26 File #test.sql
In command -
INSERT INTO FOOBAR VALUES(2)
This is referring to the line in your script where you have the INSERT statement. It's telling you an exception was raised at this point.
Error report -
This is showing the error stack:
ORA-20001: This is an illegal insertion/update
This is the actual exception that was raised.
ORA-06512: at "HR.CHECK_FOOBAR", line 11
ORA-04088: error during execution of trigger 'HR.CHECK_FOOBAR'
These are additional messages just to tell you where the exception was originally raised, in this case, in your trigger on line 11, where the RAISE_APPLICATION_ERROR is, as you might expect. Note that line numbers for triggers refer to the executable portion of the trigger, so in your case the DECLARE is line 1.

Answer 2

The ORA-04088 error means that the trigger has an un-handled exception. You are raising a application error but then not handling it. You need to handle this exception as per below.
CREATE OR REPLACE TRIGGER check_foobar
BEFORE INSERT OR UPDATE OF VarX ON FOOBAR
FOR EACH ROW
DECLARE
counter NUMBER(38);
BEGIN
SELECT count(*)
INTO counter
FROM BARFOO
WHERE VarX = :NEW.VarX
GROUP BY VarX;
IF counter > 0 THEN
RAISE_APPLICATION_ERROR(-20001, 'This is an illegal insertion/update');
END IF;
EXCEPTION
WHEN NO_DATA_FOUND THEN
DBMS_OUTPUT.PUT_LINE('TEST');
WHEN OTHERS THEN
IF SQLCODE = -20001 THEN
-- do some logging
RAISE;
ELSE
-- do some logging and any other actions you feel are needed. Then depending on needs you can raise or not.
END IF;
END;
/