Hierarchies as relationships ​

Hierarchies are such relationships: check - any relationship of hierarchy can be represented as a tuple.

Table::FO (is a member of) tables::class

You can define arbitrary relationships between objects and even establish some systematics/hierarchies for them.

If you are also involved in designing logical structures of databases (and this is often where ontological skills are needed, so I expect from some readers precisely this), then you can easily guess that specialization hierarchy inherits properties, composition hierarchy --- divides, and classification hierarchy ideally says "fill".