Non-SQL is very useful and I’m a huge fan of MongoDB but relational database is not that bad!
While searching here and there, I came up with this very interesting thing called Timber, it merges tree structure properties with relational databases.
Summary: Timber is a native it is based on the TAX tree algebra supports updates.
Timber is this XML management database, where the data is storage directly and as such, the tree structure is preserved.
More details on implementation can be found here