question about triggers

I am a beginner in writing triggers with Oracle and I have a simple question. I'll use a short example: Suppose I have a table T with attribute a1 and a2. What I want to do is the following : each time a tuple is inserted with a NULL value for attribute a1, set it to (say) 10.

My problem is : a row trigger like

  CREATE TRIGGER Set_attribute
  AFTER INSERT ON T1
  FOR EACH ROW
  WHEN ( new.a1 = NULL )
(

  UPDATE T1
  SET a1 = 10;
  )   

should be rejected because I attempt to modify a mutating table , right ? (Furthermore it is ambiguous since I don't know if it updates only the inserted tuples or some other tuples as well)

So how do you do this ?

CREATE TRIGGER Set_attribute
  AFTER INSERT ON T1
  FOR EACH STATEMENT
(

  UPDATE T1
  SET a1 = 10
  WHERE a1 = NULL ;
  )

should be accepted but it doesn't have the semantics I want since it could update tuples not inserted by the triggering SQL statement.

Thanks in advance for any help,

Dimitri Tombroff Received on Wed Oct 12 1994 - 14:46:23 CET