Expanding parent-child tree with additional tables I have readonly access to a table (Oracle database) which contains a hierarchical structure, represented via parent-child.

I need to add another table, which will expand the existing hierarchical tree.
The new table needs to add levels to the tree, including entire sub-trees.

The problem is this:
Any node in the expansion table needs to have a single parent, yet that parent can be in either the original table, or in the expansion table.

How can I accomplish this safely?

Things I thought of so far:


Giving each node in the expanded table two parent (one from each table)
Problem: data integrity!
Each node will have one parent, without knowing which table contains it
Problem: querying will be difficult.
Problem 2: same ID for two different nodes (in different tables)
Each node will have one parent, with another column specifying which table contains it
Problem: querying still difficult.
Here's an idea: build your "expansion table" with something like this:

expanded_table (id number, parent_src char(1), parent_id number, ...)


The parent_src column should indicate where the parent comes from, e.g. you could use 'O' of original, 'E' for expansion. (Add a primary key on id, of course.)

To handle the querying, use a view defined like this (pseudo-sql):

select 'O' source, o.id, 'O' parent_src, o.parent_id, [...] from original_table
union all
select 'E' source, e.id,   e.parent_src, e.parent_id, [...] from expanded_table


You can do hierarchical queries based on (source,id)/(parent_src,parend_id) pairs.

No problem with duplicate/overlapping ids, the source info disambiguates them.

Now data integrity is going to be very problematic.

You can't have check constraints that reference other tables, and I don't know of a way of building a "conditional foreign key" that would work here.
You could use triggers to make sure DML on the expanded table is coherent, but even with those and triggers on the original table (if you could do that, which doesn't seem to be the case), that would be very tricky to get right.

Having all DML to this "structure" go through PL/SQL that you control could do it, but that doesn't appear to be possible given the information in your post.