SQL & PL/SQL

Hi,

I have a table "test" with two columns value and mod_user. Currently the table has a before Update trigger to update the mod_user as system user
Now I have to modify the trigger to satisfy the below criteria

1) In an update statement If I manually set the mod_user, then it should be updated as defined in the update statement.

Eg:
UPDATE test
SET value = 1,
mod_user = 'xyz'
WHERE value = 4;

In the above scenario mod_user should be updated to 'xyz' in the table

2) If I do not mention any set clause for mod_user, then the default windows user should be updated in the table.
UPDATE test
SET value = 1
WHERE value =4;

In the above scenario mod_user should be updated as windows user in the table

The trigger statement I have used does not seem to be working.

CREATE OR REPLACE TRIGGER t_mod_test
BEFORE UPDATE OF value ON test
FOR EACH ROW
BEGIN
:new.mod_user := NVL(SYS_CONTEXT('USERENV','OS_USER'),USER);
END;

Any suggestions on how I can make the scenario working would be highly appreciated.

Thanks in advance
Das