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

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Chapter 15. Hierarchical Database Systems (IMS)
IMS from IBM is the one hierarchical database management system still in wide use today. It is stable, well defined, scalable, and very fast for what it does. In fact, there is a good chance that IMS databases still hold more data than SQL databases.
Keywords: DL/I, control blocks, data communications component, application programs, database record, segment, sequence field, data field
I am going to assume that most of the readers of this book have only worked with SQL. If you have heard of a Hierarchical Database System, it was mentioned in a database course in college and then forgotten. In some ways, that is too bad. It helps to know how earlier tools worked so that you can see how new tools evolved ...

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