Not to steal thunder from graph databases, I want to point out that recursive SQL enables support for graph-like data structures in a relational database. You

Not to steal thunder from graph databases, I want to point out that recursive SQL enables support for graph-like data structures in a relational database. You write constraint logic in the form of trigger functions to enforce directionality and prevent cycles and so forth. With this, you may not need a full graph database for your work. It's up to you to decide which is the best tool for the work, but you can't make a sound decision without being aware of the tool in the first place.