Oracle -PL-SQL-Trigger – Old and New Column Values

Within a trigger body of a row trigger, two correlation names exist for every column of the table being modified: one for the old column value, and one for the new column value. Depending on the type of triggering statement, certain correlation names might not have any meaning.

•  A trigger fired by an INSERT statement has meaningful access to new column values only. Because the row is being created by the INSERT, the old values are null.
• A trigger fired by an UPDATE statement has access to both old and new column values for both BEFORE and AFTER row triggers.
• A trigger fired by a DELETE statement has meaningful access to : old column values only. Because the row no longer exists after the row is deleted, the :new values are NULL. However, you cannot modify :new values: ORA-4084 is raised if you try to modify :new values.

 The new column values are referenced using the new qualifier before the column name, while the old column values are referenced using the old qualifier before the column name.

 For more detail refer oracle.com