Oracle PL/SQL: Loop Over Trigger Columns Dynamically

Question

Inside of a trigger I'm trying to loop over all columns on a table and compare the new values to the old values. Here is what I have so far:

CREATE OR REPLACE TRIGGER "JOSH".TEST#UPD BEFORE 
UPDATE ON "JOSH"."TEST_TRIGGER_TABLE" REFERENCING OLD AS OLD NEW AS NEW FOR EACH ROW
declare    
   oldval varchar(2000);   
   newval varchar(2000);   
begin    
   for row in (SELECT column_name from user_tab_columns where table_name='TEST_TRIGGER_TABLE') loop  
     execute immediate 'select :old.'||row.column_name||' from dual'   into oldval;  
     execute immediate 'select :new.'||row.column_name||' from dual'   into newval;  
     --Do something here with the old and new values
   end loop;  
end;

The trigger compiles, but when the trigger fires, I'm getting:

ORA-01008: not all variables bound

on the first execute immediate because it's expecting a value for :old. :old and :new are already defined as part of the trigger, but it appears that execute immediate can't see those variables.

Is there a way to dynamically iterate over the column values in a trigger?

Answer 1

No, you cannot reference :old and :new values dynamically. As Shane suggests, you can write code to generate the static trigger code, if that makes life easier. Also, you can make "do something here" into a package procedure so that your trigger becomes:

CREATE OR REPLACE TRIGGER JOSH.TEST#UPD BEFORE 
UPDATE ON JOSH.TEST_TRIGGER_TABLE
begin    
   my_package.do_something_with (:old.col1, :new.col1);
   my_package.do_something_with (:old.col2, :new.col2);
   my_package.do_something_with (:old.col3, :new.col3);
   -- etc.
end;

(You can ditch the pointless REFERENCING clause by the way).

Answer 2

I'm not sure if you can do what you are trying to do. What is the reason you don't want to explicitly name the table columns inside the PL/SQL code? If the table fields are changing often, you could build PL/SQL that dynamically builds the PL/SQL trigger for each table (with the explicit field names in each). Each time the table changes, you could run that PL/SQL to generate the new trigger.