I think Keith and I are making closely related points. It really behooves software (or most other) designers to consider function carefully before going to implementation. You probably want to create a [directed, acyclic graph](https://www.techopedia.com/definition/5739/directed-acyclic-graph-dag) (aka "DAG"), with nodes represented in a single table with constraints or procedure to avoid cycles and a convention for absence of a predecessor. This is a common problem for which I am sure you can find many solutions, including database schemas. **Off-topic side-note on horse-transport:** From my observation of horses, and from many interesting family tales of their behavior and personalities, I know that if a horse wanted to get the buggy somewhere and had a place to push on it, there would be no need for a whip and the buggy would move toward that same place.