newbie question on triggers

1. I have a table in which some columns have the "not null" clause.
2. I have written an insert trigger which is to be fired -before- insertion.
3. I have code in my trigger to test the not-null columns and, if the user has not provided data, write a message saying specifically which required field was omitted (or, at least, the first detected).
4. Yet, when I insert data into that table, with null values in the required field, I get the Oracle error message and my trigger is not fired at all.

I want to keep the field defined as "not null" for documentation reasons and to make -real- sure data integrity is observed (just in case someone with the necessary authority but not the necessary knowledge drops the trigger). I want the trigger to work for reason 3 above.

Is there any way I can do this?

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Jim Lyons                     |  Operating Systems Specialist
Computation Center            |  512-475-9331
University of Texas at Austin |  godzilla_at_seuss.cc.utexas.edu
http://www.cc.utexas.edu/~godzilla/jimlyons.html