Auditing tables..

Question

Hi Experts,
Could anyone help me with the following issue.
we are trying to implement the "audit feature" on a table. what is the best way to do the audit? (keep track of all the changes in the table)
This is what we are doing..
  Main_table
  id
  first_name
  last_name
   dob
  hire_date
  created_date
  created_by
  updated_date
  updated_by
  audit_table
  ad_action (whether it is insert,update or delete)
  ad_time (time)
  ad_user (whos is the user)
  id
  first_name
  last_name
   dob
  hire_date
  created_date
  created_by
  updated_date
  updated_by
and i have a trigger on the "main_table" like this
  CREATE OR REPLACE TRIGGER Main_table_AIUD after
insert or update or delete on Main_table for each row
begin
   declare
      ljn_action varchar2(3);
   begin
      if inserting then
         ljn_action := 'INS';
      elsif updating then
         ljn_action := 'UPD';
      else
         ljn_action := 'DEL';
      end if;
      if inserting  then
         insert into audit_table
          ad_action,
          ad_time,
          ad_user,
          id
          first_name
         last_name
         dob
         hire_date
         created_date
         created_by
         updated_date
         updated_by
         values
          ljn_action
         ,sysdate
         ,nvl(v('APP_USER') ,USER)
         :new.id,
        :new.first_name,
         :new.last_name,
          :new.dob,
         :new.hire_date,
         :new.created_date,
         :new.created_by,
         :new.updated_date,
          :new.updated_by
      elsif updating then
        insert into audit_table
          ad_action,
          ad_time,
          ad_user,
          id
          first_name
         last_name
         dob
         hire_date
         created_date
         created_by
         updated_date
         updated_by
         values
          ljn_action
         ,sysdate
         ,nvl(v('APP_USER') ,USER)
         :old.id,
        :old.first_name,
         :old.last_name,
          :old.dob,
         :old.hire_date,
         :old.created_date,
         :old.created_by,
         :old.updated_date,
          :new.updated_by
      else
         insert into audit_table
          ad_action,
          ad_time,
          ad_user,
          id
          first_name
         last_name
         dob
         hire_date
         created_date
         created_by
         updated_date
         updated_by
         values
          ljn_action
         ,sysdate
         ,nvl(v('APP_USER') ,USER)
         :old.id,
        :old.first_name,
         :old.last_name,
          :old.dob,
         :old.hire_date,
         :old.created_date,
         :old.created_by,
         :old.updated_date,
          :new.updated_by
      end if;
   end;
end;
/For "Insert" and "Delete" as i need to capture all the elements my code would be ok.
But for "update" how can i capture only the columns that got changed? and put it in the audit table
so next time if i go to the audit table i can clearly see that these elements got changes.
Is there any better way to do this?
Please let me know
Thanks

Answer 1

Yeah for tracking OLD and NEW values you have to write a trigger.
Your trigger code seems fine except a few quirks.
You audit_table should contain two fields against each column of the main table. e.g. for first_name column in main_table there should be two fields old_firstname and new_firstname in the audit_table.
And then each section of trigger (inserting, updating, deleting) insert both :OLD and :NEW values to the table. In case of INSERT there will be no :OLD values so just insert NULL and in case of DELETE there will be no :NEW values so insert NULL. But in case of UPDATE its important to track both :NEW and :OLD.
and if you track both NEW and OLD values in the trigger and put them in the audit_table then you no longer need to identify which columns are changed. You can figure that later on when you query the audit_table, any fields where OLD and NEW values are same was not updated.
If you still want to check which column was changed in the trigger, refer to Himanshu Kandpal's post above.
And you probably don't need this code in the trigger:
      if inserting then
         ljn_action := 'INS';
      elsif updating then
         ljn_action := 'UPD';
      else
         ljn_action := 'DEL';
      end if;You can Just use the values 'DEL','INS' and 'UPD' where appropriate instead of the ljn_action variable.