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 the way IMS could (see Chapter 12), 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. E. F. Codd described a method for showing hierarchies in SQL that consisted of a column for the boss and another column for the employee in the relationship. It was a direct implementation in a table of the Adjacency List Model of a graph. Oracle was the first commercial database to use SQL, and the sample database that comes with their product, nicknamed the “Scott/Tiger” ...
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