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Navigational databases

The original database management systems were developed by legendary computer scientists such as Charles Bachman, who gave a lot of thought to the way software should be built in a world of extremely scarce computing resources. Bachman invented a very natural (and as we will see later, graphical) way to model data: as a network of interrelated things. The starting point of such a database design was generally a Bachman diagram (refer to the following diagram), which immediately feels like it expresses the model of the data structure in a very graph-like fashion:

Bachman diagram

These diagrams were the starting points for database management systems that used either networks or hierarchies as the basic structure for their data. Both the network databases and hierarchical database systems were built on the premise that data elements would be linked together by pointers:

An example of a navigational database model with pointers linking records

As you can probably imagine from the preceding discussion, these models were very interesting and resulted in a number of efforts that shaped the database industry. One of these efforts was the Conference on Data Systems Languages, better known under its acronym, CODASYL. This played an ever so important role in the information technology industry of the sixties and seventies. It shaped one of the world's dominant computer programming systems (COBOL), but also provided the basis for a whole slew of navigational databases such as IDMS, Cullinet, and IMS. The latter, the IBM-backed IMS database, is often classified as a hierarchical database, which offers a subset of the network model of CODASYL.

Navigational databases eventually gave way to a new generation of databases, the Relational Database Management Systems. Many reasons have been attributed to this shift, some technical and some commercial, but the main two reasons that seem to enjoy agreement across the industry are as follows:

The complexity of the models that they used. CODASYL is widely regarded as something that can only be worked or understood by absolute experts--as we partly experienced in 1999, when the Y2K problem required many CODASYL experts to work overtime to migrate their systems into the new millennium.
The lack of a declarative query mechanism for navigational database management systems. Most of these systems inherently provide a very imperative approach to finding data: the user would have to tell the database what to do instead of just being able to ask a question and having the database provide the answer.

This allows for a great transition from navigational to relational databases.