Database Reference
In-Depth Information
From a conceptual standpoint, all relationships exist between exactly
two entities. Logically, we have a relationship between Schools and Deans.
Technically, you could leave the notation with these two entities showing
that there are two one-to-many relationships, one in each direction.
Alternatively, you can show a single relationship that shows a “many” at
both ends. However, from a practical standpoint, it may be easier to use a
third entity to show the relationship, as shown in Figure 2.6.
F
IGURE
2.6
The Schools and Deans entities, many-to-many relationship with
third entity
Arguably, this is a violation of the ideal that a logical model contain no
elements of physical implementation. The use of a third entity, whereby we
associate Deans and Schools by ID, duplicates the physical implementa-
tion method for many-to-many relationships. Physically, it is impossible to
model this relationship without using a third table, sometimes called a
junction
or
join
table. So using it in the model may not conform to strict
logical modeling guidelines; however, adding it in the logical model can
help remind you why the relationship is there, as well as aid future model-
ers in understanding the relationship in the logical model.
Additionally, using the third entity can let you use additional attributes
that actually describe each instance of the relationship. For example, you
can use the Deans_Schools entity to note how long a specific dean has
been in charge of a given school. If you have multiple combinations, the
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