Database Reference
In-Depth Information
The ERD is normally used to show binary relationships but can also show n-ary
relationships. In many cases, E-R diagrams show only binary relationships. For example,
a possible ternary relationship not shown in Figure
3-4b
is
Supplier-Schedule
(linking
Supplier
,
InventoryItem
and
Project
). The reason for this is the following principle:
The proof for this principle is beyond the scope of this course. However, we shall revisit
it later, and provide additional clarifications. For now, a simple illustration will suffice:
Figure
3-5
shows how the ternary
Supplier-Schedule
relationship may be broken down
into three binary relationships. Since care must be taken in applying this principle, it will be
further discussed in the next chapter.
Figure 3-5.
Decomposing a Ternary Relationship
