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Joe Celko's Trees and Hierarchies in SQL for Smarties, 2nd Edition by Joe Celko

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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.
In a short paper, Dr. ...

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