Question

I want to prevent the database from storing any values bigger than 20 into a table.

CREATE OR REPLACE TRIGGER Dont_Allow
AFTER INSERT ON Cities
FOR EACH ROW

WHEN (new.IDCity > 20)

BEGIN
   dbms_output.put_line('  Failed to insert ' || :new.IDCity);
   delete from orase where IDCity=:new.IDCity;
END;

While this does work in terms of not actually adding anything with an ID > 20, every time the trigger tries to do its magic, this shows up:

ORA-04091: table SYSTEM.ORASE is mutating, trigger/function may not see it
ORA-06512: at “SYSTEM.DONT_ALLOW”, line 6
ORA-04088: error during execution of trigger ‘SYSTEM.DONT_ALLOW’

What’s a proper way of doing what I want?

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EDIT:

I’ve decided to use a trigger for this:

After a new row is inserted into Employees, a trigger checks the new guy’s salary and if it’s above 21 units / hour, it takes 5% off management’s bonus. Lame, but hey – I’m using a trigger to solve a problem I don’t have: the outcome won’t be pretty.

CREATE OR REPLACE TRIGGER Bite_Bonus
AFTER INSERT ON Employees
FOR EACH ROW

WHEN (new.HourSalary > 20)

BEGIN
   update Management set Bonus = Bonus - 5/100 * Bonus;
END;

Answer 1

You shouldn’t be using a TRIGGER for that, you should be using a CHECK, like CONSTRAINT city_id_below_20 CHECK (IDCity < 20). You can use ALTER TABLE ADD CONSTRAINT to put it on an existing table.