Is it possible to change a delete to an update using triggers? - sql

Question

Is there any way in Firebird to execute an UPDATE instead a DELETE through a trigger?
This is possible in Microsoft SQL Server by declaring the triggers as "INSTEAD".
The problem is that we have an application that uses a Firebird database and we want to prevent the deletes of records and mark then as "deleted" (a new field), but without showing any error to the user, and "cheating" the app.

Answer 1

You cannot do this with tables, but you can do it with views. Views can have triggers on insert, update and delete that modify the underlying table(s). See also Updatable Views in the Firebird 2.5 Language Reference.
In short, create a table for the data, add a view, add triggers that insert/update/delete through the view to the underlying table. Users can then use the view as if it is a table.
An example
I'm using Firebird 3, but this will work with minor modifications in Firebird 2.5 and earlier.
A table example_base:
create table example_base (
id bigint generated by default as identity constraint pk_example_base primary key,
value1 varchar(100),
deleted boolean not null default false
)
A view example:
create view example (id, value1)
as
select id, value1
from example_base
where not deleted;
Do not create the view with with check option, as this will disallow inserts as the absence of the deleted column in the view will prevent Firebird from checking the invariant.
Then add an insert trigger:
create trigger tr_example_insert before insert on example
as
begin
if (new.id is not null) then
-- Don't use identity
insert into example_base(id, value1) values (new.id, new.value1);
else
-- Use identity
-- mapping generated id to new context
-- this way it is available for clients using insert .. returning
insert into example_base(value1) values (new.value1)
returning id into :new.id;
end
The above trigger ensures the 'by default as identity' primary key of the underlying table is preserved, and allows insert into example .. returning to report on the generated id.
An update trigger
create trigger tr_example_update before update on example
as
begin
-- Consider ignoring modification of the id (or raise an exception)
update example_base
set id = new.id, value1 = new.value1
where id = old.id;
end
The above trigger allows modification of the primary key; you may want to consider just ignoring such a modification or even raising an exception.
And finally a delete trigger:
create trigger tr_example_delete before delete on example
as
begin
update example_base
set deleted = true
where id = old.id;
end
This trigger will mark the record in the base table as deleted.
To use this, just grant your users select, insert and update privileges to the view (and not the table).
The only caveat I'm aware of is that defining foreign keys will need to point to example_base, not to example, and the behavior of foreign keys will be slightly off. The record in the base table will continue to exist, so the foreign key will not block deletion. If that is something that is necessary, you will need to emulate constraint behavior (which could be tricky).

Answer 2

YES! It can be made on VIEWs.
That's the way I solved it.
If a View has a trigger, then the trigger is the responsible of making the real update or delete on the underlying table.... So... a DELETE trigger that makes an UPDATE to the table solved my problem.