How can I retrieve the condition in an update or delete using instead of trigger

Question

I have 2 tables, on which a view is created. To distinguish the tables within the view trigger, a column "source" had been added in the view creation. I want to be able to delete row(s) in table t1 or table t2 through the view, depending on a condition. The problem is that I don't know how I can get back the delete condition IN the INSTEAD of trigger :
CREATE TABLE t1(m NUMBER, n VARCHAR2(10), o date);
CREATE TABLE t2(a NUMBER, b VARCHAR2(10), c date);
-- The view is created based on the 2 tables.
CREATE OR REPLACE VIEW vt12 AS
SELECT 't1' source, m, n, o
FROM t1
UNION
SELECT 't2' source, a, b, c
FROM t2;
-- The trigger will fire whenever INSERT is performed on the tables through the view
create or replace trigger tvt12 instead of insert or delete on vt12
begin
if inserting then
if :new.source = 't1' then
insert into t1 values (:new.m, :new.n, :new.o);
else
-- will insert a default value of -1 for the fourth column "d"
insert into t2(a,b,c,d) values (:new.m, :new.n, :new.o, -1);
end if;
elsif deleting then
-- We don't know the condition for the deletion, so all rows from the
-- table from which the current row is coming will be deleted
if :old.source = 't1' then
delete t1;
else
delete t2;
end if;
end if;
end;
show error
insert into vt12 values ('t1',1,'1',sysdate);
insert into vt12 values ('t2',2,'2',sysdate+1);
insert into vt12 values ('t1',3,'3',sysdate+2);
insert into vt12 values ('t2',4,'4',sysdate+3);
insert into vt12 values ('t1',5,'5',sysdate+4);
insert into vt12 values ('t2',6,'6',sysdate+5);
commit;
select * from vt12;
select * from t1;
select * from t2;
delete vt12 where m = 1;
commit;
select * from vt12;
select * from t1;
select * from t2;
When I execute this script, the delete statement seems to recognize that the condition is affecting a row in table t1, and therefore deletes all rows from the table, as specified in the delete statement of the trigger... But, what I want to do is ONLY to delete the row affected by the original condition. So my question is to know how I can get back the original condition for the delete, and still use it in the trigger, that will be executed INSTEAD of the delete statement.. I checked in the doc, and everywhere an example of INSTEAD OF trigger can be found, but it's always and only using an INSTEAD OF INSERT trigger, which doesn't need a condition.
Can anyone help ?
null

Answer 1

I've never faced this case myself, but from the documentation it would seem that the INSTEAD OF DELETE is also a FOR EACH ROW trigger. Therefore, you don't really have to worry about the actual "where" clause of the original "delete" operation: Oracle has it all parsed for you, so you just need to redirect, in turn, each view record being deleted to the appropriate table.
In extenso: for your
delete vt12 where m = 1;
=> the INSTEAD OF trigger fires once for all records in vt12 where m=1, and I would guess that you trigger code should look like:
begin
delete from t1 where m = :old.m and n = :old.n and o = :old.o;
delete from t2 where a = :old.m and b = :old.n and c = :old.o;
end;
It's a bit of overkill compared to the "where" clause of the original "delete" statement, but it should do the job...
BTW, if by any luck you do have some primary key on tables t1 and t2 (say: columns o and c respectively), then obviously you can make the thing simpler:
begin
delete from t1 where o = :old.o;
delete from t2 where c = :old.o;
end;
HTH - Didier