Appendix. Appendix B Hierarchical and Network Databases

For the majority of the discussions in this book we have been considering the relational model of data, first proposed by Edgar F. Codd (1970) when talking about the way that data is structured in databases. In the relational model, data is logically stored in relations, which are sets of tuples, with each tuple being a set of attribute–value pairs. In the physical embodiment of this model, with database management systems based on the SQL database language, the data is logically considered to be stored in tables.

Although it is the most common logical construct for storing data in databases, the table is not the only logical construct. Other models of data include:

• the hierarchical model;

• the network model;

• the multidimensional model;

• the object-oriented model.

Both the multidimensional model of data and the object-oriented model were discussed in Chapter 12. The hierarchical model of data and the network model of data are discussed in this appendix. Both these models predate the relational model of data, which was proposed by Edgar Codd to counter the shortcomings of these two models of data.

To provide a comparison, Figure B.1 shows a conceptual data model that will be developed into the hierarchical and network schemas and Figure B.2 shows some specimen data arranged as if it was stored in a relational database.