Oracle view not updatable, advice on Instead Of triggers - oracle

Question

after migrating a system/database we modified a central table which has been used for interfacing with 15 different systems. We used this migration to add and delete a few fields in this table.
To maintain direct compatibility with the interfacing systems (i.e. only need to change the database-link), a view has been created which shows the exact same columns as the old table had. However, some of these columns are only emulated, so the view contains constructs like these:
(...)
CREATE OR REPLACE VIEW STAFF_DATA_COMPAT AS
SELECT
NVL(knownas_surname,surname) as surname,
first_name
middle_name as mid-name
NULL as ni,
NULL as home_tel_no,
(...)
Obviously, this view is not inherently updatable.
I do understand, that you need INSTEAD OF triggers for all DML (insert, update, delete) statements.
I can see, that a INSTEAD OF INSERT trigger should be quite straightforward (just inserting :NEW.field to the real table, where appropriate and ignoring the others).
But the actual question: How to write the according INSTEAD OF UPDATE/DELETE triggers? For instance, how do I take over the "WHERE" clause of an original DELETE statement? Is there anything else I should worry about, any side-effects when using these triggers?
Btw. It's Oracle 11g.

Answer 1

The INSTEAD OF trigger would look like this (I've assumed you have a primary key column id):
SQL> CREATE OR REPLACE TRIGGER trg_staff_data_cpt_instead_upd
2 INSTEAD OF UPDATE ON staff_data_compat
3 FOR EACH ROW
4 BEGIN
5 UPDATE staff_data_compat_t
6 SET knownas_surname = :new.surname,
7 first_name = :new.first_name,
8 middle_name = :new.mid_name
9 WHERE id = :new.id
10 END;
11 /
Trigger created
Note that some columns may in fact be updatable in the original view. Query the all_updatable_columns view (before creating the trigger) to find out:
SQL> CREATE TABLE staff_data_compat_t AS
2 SELECT object_name knownas_surname,
3 owner surname,
4 object_type first_name,
5 subobject_name middle_name
6 FROM all_objects;
Table created
SQL> CREATE OR REPLACE VIEW staff_data_compat AS
2 SELECT
3 NVL(knownas_surname,surname) as surname,
4 first_name,
5 middle_name mid_name,
6 NULL as ni,
7 NULL as home_tel_no
8 FROM staff_data_compat_t;
View created
SQL> SELECT * FROM all_updatable_columns WHERE table_name = 'STAFF_DATA_COMPAT';
OWNER TABLE_NAME COLUMN_NAME UPDATABLE INSERTABLE DELETABLE
------ ------------------ ------------ --------- ---------- ---------
VNZ STAFF_DATA_COMPAT SURNAME NO NO NO
VNZ STAFF_DATA_COMPAT FIRST_NAME YES YES YES
VNZ STAFF_DATA_COMPAT MID_NAME YES YES YES
VNZ STAFF_DATA_COMPAT NI NO NO NO
VNZ STAFF_DATA_COMPAT HOME_TEL_NO NO NO NO
If you only need to insert/update these columns, you don't need an INSTEAD OF trigger.

Answer 2

INSTEAD OF triggers are implicitly "FOR EACH ROW", so you don't have to find out the WHERE clause, you just do something like this:
begin
delete base_table
where pk = :old.pk;
end;
This also shows one of the drawbacks of INSTEAD OF triggers: they work row-by-row not in sets.