ELEMENTS USED IN LOGICAL DATA MODELS - A Developer's Guide to Data Modeling for SQL Server - page 38

You can see that there is a relationship between the entities such that
you might have more than one dean for each school. This relationship is in-
herently scalable, because the separate entities can be updated and man-
aged independently.

Many-to-Many Relationships
Of the logical relationships, many-to-many relationships, also called non-
specific relationships, are the most difficult concept, and possibly the most
difficult to design. To simplify, in a many-to-many relationship the objects
in an entity can be related to more than one object in a secondary entity,
and the secondary objects can be related to more than one object in the
initial entity. Imagine auto parts, specifically something simple like seats.
Any given vehicle probably has more than one type of seat, perhaps two
bucket seats for the front passenger and driver and a single bench seat in
the rear. However, automakers almost always reuse seats in multiple mod-
els of vehicles. So, as entities, Seats can be in multiple Vehicles, and
Vehicles can have multiple Seats.
Back to our university. What if the decision is made that a single dean
can manage multiple schools or even that one school can have more than
one dean? In Figure 2.5, we've arranged the Schools and Deans entities so
that either entity can have multiple links to the other entity.

F IGURE 2.5

The Schools and Deans entities, many-to-many relationship