Chapter 2. Adjacency List Model
In the early days of System “R” at IBM, one of the arguments against a relational database was that SQL could not handle hierarchies like IMS could and would therefore not be practical for large databases. It might have a future as an ad hoc query language, but that was the best that could be expected of it.
Keywords: subtree, anomalies, NULLs, navigation, traversal, self-joins, aggregation
In the early days of System “R” at IBM, one of the arguments against a relational database was that SQL could not handle hierarchies like IMS could and would therefore not be practical for large databases. It might have a future as an ad hoc query language, but that was the best that could be expected of it.

Get Joe Celko's Trees and Hierarchies in SQL for Smarties, 2nd Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.