Back in the days before PCs, when I worked as a Systems Analyst on IBM Mainframes you had the choice of storing your data either in a hierarchical database (IMS/DL1) or on the new-fangled relational DMBS (DB2/SQL).

If your data model was heirarchical, you didn't attempt to implement it on DB2, conversely if the data model was relational, you didn't use IMS...

30 years later we seem to have lost sight of the fact that not *all* data models are relational.