Handling mod_user in a table using trigger

Question

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

Answer 1

It sounds like you want something like
SQL> create table test(
  2    value number,
  3    mod_user varchar2(100)
  4  );
Table created.
SQL> create or replace trigger t_mod_test
  2    before update on test
  3    for each row
  4  begin
  5    if( updating( 'MOD_USER' ) )
  6    then
  7      null;
  8    else
  9      :new.mod_user := NVL( sys_context('USERENV', 'OS_USER')
10                           ,user );
11    end if;
12  end;
13  /
Trigger created.
SQL> ed
Wrote file afiedt.buf
  1  create or replace trigger t_mod_test
  2    before update on test
  3    for each row
  4  begin
  5    if( not updating( 'MOD_USER' ) )
  6    then
  7      :new.mod_user := NVL( sys_context('USERENV', 'OS_USER')
  8                           ,user );
  9    end if;
10* end;
SQL> /
Trigger created.
SQL> insert into test values( 4, 'xyz' );
1 row created.
SQL> update test
  2     set value = 1
  3        ,mod_user = 'abc'
  4   where value = 4;
1 row updated.
SQL> select * from test;
     VALUE MOD_USER
         1 abc
SQL> update test
  2     set value = 2
  3   where value = 1;
1 row updated.
SQL> select * from test;
     VALUE
MOD_USER
         2
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