Database Reference
In-Depth Information
Appendix A
Legacy System Models:
Hierarchical and Network
Chapter 1 briefly introduced the hierarchical and network data models. These data
models were widely used before the adoption of the relational data model. The
earlier generation of data systems composed of these two data models are gener-
ally known as legacy system models. The information legacy in many enterprises is
still residing on databases supporting these two data models.
As you know, the relational data model is the one that is widely used now because
of its superiority and strength. Nevertheless, you need to review the earlier data
models for two reasons. First, hierarchical and network databases are still in use.
Second, a brief review of the earlier data models will enhance your appreciation of
the relational model and its superior features.
In this appendix, we will summarize the basic concepts of the two legacy system
models and provide a mapping of the semantic data model with each of the two
models.
HIERARCHICAL DATA MODEL
This model is the oldest of the database models. Unlike the relational data model
or the network data model, the hierarchical data model did not originate with any
systematic document or research results. The network model is based on the stipu-
lations by the DBTG of CODASYL. On the other hand, the relational model
originated from the research specifications of Dr. Codd.
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