---------- Original Message -----------
From: "Rick Debay" <rdebay@...>

> I have a parent table with multiple child tables.
> Each child row has a foreign key that points to one row in the parent.
> Each parent row must have a child row pointing to it.
> Each parent row can have only child from any of the child tables
> pointing to it.
> The child is created before the parent.

------- End of Original Message -------

Using a constant tag to ensure a parent has children only in one child table at
a time:

alter table parent add child_type char(1) check (value in ('A', 'B', 'C'));
alter table child_a add child_type char(1) check (value = 'A') default 'A';
/* repeat */
alter table child_a add constraint fk_a foreign key (parent_id, child_type)
references parent (id, child_type) on update cascade on delete set null;
/* repeat */

You already have example triggers for ensuring the parent gets created when the
child is created, preventing duplicates via the insert statement; but if you
want to allow parentless children, that doesn't seem quite right. You'd have to
create the child, then re-delete the parent? What about updating a child's FK
to the parent, such that it points to another pre-existing parent (with
children in another table)?

No childless parents:

create global temporary table altered_parents (
parent_id integer not null primary key
);

create trigger track_parents after insert or update or delete on parent as
begin
if (inserting or updating) then
update or insert into altered_parents (parent_id) values (new.id);
if (deleting or updating) then
update or insert into altered_parents (parent_id) values (old.id);
end

create trigger track_parents after insert or update or delete on child_a as
begin
if (inserting or updating) then
update or insert into altered_parents (parent_id) values (new.parent_id);
if (deleting or updating) then
update or insert into altered_parents (parent_id) values (old.parent_id);
end
/* repeat */

create exception childless_parent 'You are not allowed to create a childless
parent';
create trigger check_parents on transaction commit as
begin
if (exists(select * from altered_parents inner join parent on parent.id =
altered_parents.parent_id /* once a parent is deleted, we no longer care to
check it */ where not exists (select * from child_a where child_a.parent_id =
altered_parents.parent_id) and not exists (...) and not exists(...)))
/* you could use the same technique to catch multi-children parents, here,
without 'tagging' the rows as above */
exception childless_parent;
delete from altered_parents; /* I normally put the body of these triggers in a
procedure I can call 'as i go', with a final double-check at transaction
commit; in that case, you only want to re-check what hasn't been previously
checked. */
end

--Philip