Database Reference
In-Depth Information
ATTRIBUTES
In the discussion on entities and entity types, you must have observed that a group
of entities were put together and called an entity type because these entities are
similar. Customer entities are grouped together as the entity type CUSTOMER for
the reason that the entities share common characteristics. If one customer entity
possesses the characteristics of name, address, and phone, then another customer
entity also has these characteristics; yet another customer entity also shares these
characteristics. Common characteristics or attributes determine whether a number
of entities or “things” may be grouped together as one type.
The E-R data model represents inherent characteristics of entity types as attrib-
utes in the same way as the object-based data model indicates properties of object
sets. Therefore, our discussion of attributes can be brief, just highlighting the rep-
resentation and serving as a refresher.
Attribute Specification
Inherent characteristics or basic properties of an entity type are defined as attrib-
utes for that entity type. What is the function of attributes? Attributes describe an
entity type. Consider the attributes for the entity type CUSTOMER shown in
Figure 7-6.
The figure presents four attributes for CUSTOMER. Look at the following
values of the attributes for one customer or a single entity:
CustomerName:
John A. Harreld
CustomerAddress:
2401 Merry Boulevard, Branchburg, NJ 08810
CustomerPhone:
(908) 722-8419
CreditCode:
AAB
The values indicate the particular customer entity you are interested in.
The values describe the specific customer and distinguish that customer from
others.
Similar to the attributes represented in an object-based data model, the attrib-
utes in an E-R data model also possess the following features:
CustomerAddress
CustomerName
CUSTOMER
CustomerPhone
CreditCode
Figure 7-6
Attributes for CUSTOMER entity type.
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