how to create a trigger in oracle which will restrict insertion and update queries on a table based on a condition

Question

I have account table as this--

create table account
(
acct_id int,
cust_id int,
cust_name varchar(20)  
)

insert into account values(1,20,'Mark');
insert into account values(2,23,'Tom');
insert into account values(3,24,'Jim');

I want to create a trigger which will ensure that no records can be inserted or update in account table having acct_id as 2 and cust_id as 23.

My code is --

create trigger tri_account
before insert or update
on account
for each row
begin
    IF (:new.acct_id == 2 and :new.cust_id == 23) THEN 
        DBMS_OUTPUT.PUT_LINE('No insertion with id 2 and 23.');
rollback;
    END IF; 
end;

so this trigger is created , but with compilation error. now when I insert any record with acct_id as 2 and cust_id as 23,it doesent allow. But I get an error saying ORA-04098: trigger 'OPS$0924769.TRI_ACCOUNT' is invalid and failed re-validation

I don't understand this.I also want to show a message that dis insertion is not possible. please Help...

Answer 1

1. The equality operator in Oracle is =, not ==.
2. You cannot commit or rollback in a trigger. You can throw an exception which causes the triggering statement to fail and to be rolled back (though the existing transaction will not necessarily be rolled back).
3. It does not appear that this trigger compiled successfully when you created it. If you are using SQL*Plus, you can type show errors after creating a PL/SQL object to see the compilation errors.
4. You should never write code that depends on the caller being able to see the output from DBMS_OUTPUT. Most applications will not so most applications would have no idea that the DML operation failed if your trigger simply tries to write to the DBMS_OUTPUT buffer.

Putting those items together, you can write something like

create trigger tri_account
before insert or update
on account
for each row
begin
    IF (:new.acct_id = 2 and :new.cust_id = 23) THEN 
        raise_application_error( -20001, 'No insertion with id 2 and 23.');
    END IF; 
end;