It would be a great blessing to see extensions to the SQL standard to handle trees more gracefully. Each entity type maps to one table. I urge practitioners to do themselves a favor and stay away from Joe Celko's pronouncements. I have never encountered anything said or written by him that is not confused, irrelevant, misleading, or riddled through with errors. He simply does not understand relational technology and, therefore, whatever he understands about SQL is highly suspect.
Hierarchies are common, but nowhere near as common as programmers think.
Furthermore, when hierarchies truly exist in the real world, they can be represented and queried much better relationally. Relational technology was, in fact, developed as an alternative to hierarchic DBMSs, which were discarded decades ago as not cost-effective. February 21, 2001.