How to call a trigger when constraint violation occurs? - sql

Question

I have a situation where users are not allowed to enter a duplicate value. If user tries to add duplicate value, the system saves a details of user in a audit table. Trigger is used for that. My code is below
create or replace trigger tr_add_on_audit_table
before insert on lds_consultant
for each row
declare
uname varchar2(30);
begin
select username into uname from lds_consultant where username = :NEW.USERNAME;
if uname <> '' or uname <> null then
insert into audit_table values(null, null, 'nishan', 'insert', null, null, 'cmd', null, 'LDS_CONSULTANT', 'CONSULTANT_ID',null, null, null);
end if;
end;
but this code doesn't insert data into audit table.
How can I achieve that?

Answer 1

NULL isn't equal to nor different from anything. You should use IS NULL or IS NOT NULL, not <> nor =.
Something like this:
create or replace trigger tr_add_on_audit_table
before insert on lds_consultant
for each row
declare
uname varchar2(30);
begin
select username
into uname
from lds_consultant
where username = :NEW.USERNAME;
if uname is not null then --> this!
insert into audit_table
values(null, null, 'nishan', 'insert', null, null, 'cmd', null, 'LDS_CONSULTANT', 'CONSULTANT_ID',null, null, null);
end if;
exception
when no_data_found then
null;
end;
I included exception handler section in case that SELECT doesn't return anything; if it isn't probable, remove it (or handle it properly; I'm doing nothing (NULL;). Also, handle other exceptions, if necessary.
Also, I'd suggest you to name all columns you're inserting into. Today, you know what value goes where, but in a matter of a month or two you'll forget what is the third NULL value supposed to mean.
Furthermore, you said that user isn't allowed to enter a duplicate value - well, this code won't make it happen.
The simplest option is to create a unique key constraint on the USERNAME column and let Oracle handle duplicates.
If you want to do that yourself, you should e.g.
raise_application_error(-20000, 'Duplicate username is not allowed);
However, that won't save your INSERT into the table as everything will be rolled back. In order to fix that, create a procedure that uses pragma autonomous_transaction and commits insert into the audit table.
Everything would look like this:
create or replace procedure p_audit as
pragma autonomous_transaction;
begin
insert into audit_table
values(null, null, 'nishan', 'insert', null, null, 'cmd', null, 'LDS_CONSULTANT', 'CONSULTANT_ID',null, null, null);
commit;
end;
/
create or replace trigger tr_add_on_audit_table
before insert on lds_consultant
for each row
declare
uname varchar2(30);
begin
select username
into uname
from lds_consultant
where username = :NEW.USERNAME;
if uname is not null then
p_audit;
raise_application_error(-20000, 'Duplicates are not allowed')
end if;
exception
when no_data_found then
null;
end;
/
But, once again, why bother? Uniqueness is the keyword here.