trigger to override the message to error(ORA-02292) - sql

Question

I want override the message that generate error (ORA-02292).
It is message like that
ORA-02292: integrity constraint (IVANKA.FK_SUPPLIER) violated - child record found
I want a trigger to override the above message to his example on this (MY override :))
I tried do like that
for first create table
CREATE TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50),
CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)
);
CREATE TABLE products
( product_id numeric(10) not null,
supplier_id numeric(10) not null,
CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id)
REFERENCES supplier (supplier_id)
);
then insert data
INSERT INTO supplier
(supplier_id, supplier_name, contact_name)
VALUES (1000, 'Microsoft', 'Bill Gates');
INSERT INTO products
(product_id, supplier_id)
VALUES (50000, 1000);
then do trigger
create or replace trigger sup_z
after delete on supplier
for each row
declare
v_error_constraint exception;
pragma exception_init(v_error_constraint, -2292);
begin
null;
exception
When v_error_constraint then
raise_application_error(-20001,
'My ovvervide:)');
End;
then do delete to generate message
DELETE from supplier
WHERE supplier_id = 1000
but I see not my message in trigger I see
ORA-02292: integrity constraint (IVANKA.FK_SUPPLIER) violated - child record found
Can you help me? What am I doing wrong?

Answer 1

Your trigger is fired after the DELETE has been done, so it will never fire if the DELETE gives an error.
You could use a BEFORE trigger to avoid the DELETE, if you find some child records; for example:
create or replace trigger sup_z
before delete on supplier
for each row
declare
vNumberOfChild number;
begin
select count(1)
into vNumberOfChild
from products
where supplier_id = :old.supplier_id;
--
if vNumberOfChild > 0 then
raise_application_error(-20001, 'My ovveride:)');
end if;
End;
Another way could be defining a procedure to handle the delete:
SQL> create or replace procedure deleteSupplier(p_id in numeric) is
2 vNumberOfChild number;
3 begin
4 select count(1)
5 into vNumberOfChild
6 from products
7 where supplier_id = p_id;
8 --
9 if vNumberOfChild > 0 then
10 dbms_output.put_line('My ovveride');
11 else
12 delete supplier
13 where supplier_id = p_id;
14 end if;
15 end;
16 /
Procedure created.
SQL> set serveroutput on
SQL> exec deleteSupplier(1000);
My ovveride
PL/SQL procedure successfully completed.
This avoids running the delete checking the data before, so you need no triggers.

Answer 2

Check this out (Display custom message when constraint is violated PL/SQL). I think you want something similar.

Answer 3

When all you need is just the message to be printed, why do we need a trigger in the first place? Shouldn't we be handling this just with an exception alone?
declare
v_error_constraint exception;
pragma exception_init(v_error_constraint, -2292);
begin
DELETE from supplier
WHERE supplier_id = 1000;
exception
When v_error_constraint then
raise_application_error(-20001,
'My ovvervide:)');
End;