Fig. 7.3.8 Different levels of modelling.

In Fig. 7.3.8, it is seen that the ERD is the equivalent to a globe of the world. The dis is
the equivalent to the map of Texas. And the physical database design is the equivalent of
the city map of Dallas, Texas. The globe—the ERD—is complete but not detailed. The
map of Texas—the dis—is incomplete in that you can’t find your way to and from
Chicago with a map of Texas. But the map of Texas has a great deal more detail than the
globe. The city map of Dallas—the physical data model—is even less complete. You
cannot find your way from El Paso to Midland with a city map of Dallas. But you have
even more details in the city map of Dallas than you do in the state map of Texas.

An Example of the Linkage

The complete linkage of the different forms of data modeling to each other is shown in
Fig. 7.3.9.

Chapter 7.3: Data Modeling for the Structured Environment