What is the best way to model user defined hierarchical relationships in a database?

Question

Essentially, I want the user to be able to define a hierarchical model, but then I need to allow the user to store data within their defined model. Does this make sense? So the

Answer 1

You need to implement three concepts:

• the "unit types" and their allowed associations
• the hierarchy
• the actual units

These concepts can coexist more or less independently in the model, but work together.

create table unittype
(
    id int;
    name varchar(20);
)

create table unitrelationship
(
    id int;
    parent_id int;
)

You could model the hierarchy as self-referencing table:

create table hierarchy
(
    id int;
    parent_id int;
    unit_type_id int;
    unit_id int;
)

You can then have your unit instances in one or more tables and do with them what you described.

create table unit
{
    id int;
    ....
}

The good news is that you are constraining only the allowed parent types, which can be easily enforced in a user interface, for instance by picking the parent from a list of all existing units of the allowed type.