Web-DSS-Chapter-03

38 CHAPTER 3 ■ Entity-Relationship Modeling
Name
StudentID
Address
STUDENT
Figure 3.4 The key attribute.
A composite key must be minimal in the sense that no subset of a composite key can form
the key of the entity instance. For example, if a composite key has four attributes, A1 to A4, then
any subset, say A2, A4 or A2, A3 (or any of 16 combinations), should not form a key for an
entity. In other words, we need all attributes, A1–A4, to identify each instance of an entity
uniquely. In the E-R diagram, we underline each attribute in the composite key.
For example, consider the CITY entity type (see Figure 3.5). This category includes, potentially,
all the cities in the United States. Notice that none of the attributes (i.e. Name, State or
Population) can serve as a key attribute since there are many cities in each state and two cities
could possibly have the same name or population. However, the composite attribute {Name,
State} is a valid key attribute for the CITY entity as no two cities within a state can have the same
name.
An entity can have more than one attribute that qualifies to be an identifier. For the entity
shown in Figure 3.6, each of the attributes Name, StateAbbr, and UnionOrder (the order in
which the state entered the union of the United States) can be an identifier. In this case, it is a
matter of preference as to which attribute is made an identifier or key attribute.
State
Name
Population
CITY
Figure 3.5 The composite key attribute.