A tree is a graph without cycles. A graph is a relational structure E(a,b) which can easily be stored in tables. This is why relational databases like SQL store

A tree is a graph without cycles. A graph is a relational structure E(a,b) which can easily be stored in tables. This is why relational databases like SQL store tables. So there's nothing exceptional about storing graphs, and adjacency lists or matrices are all fine. To connect with the other threads, SQLite would be probably the best candidate proposed here to store a tree.