Database Reference
In-Depth Information
Certain “things” are important to the organization. List these things and derive
these as entities of DEPARTMENT, EMPLOYEE, CUSTOMER, ORDER,
and INVOICE.
Each of these “things” has properties. Mark these properties or characteristics
and derive these as attributes of each of the entities DEPARTMENT,
EMPLOYEE, CUSTOMER, ORDER, and INVOICE. For example, note that
EMPLOYEE has characteristics such as EmployeeName, EmployeeAddress,
and SocSecNumber. Derive these as attributes of EMPLOYEE.
Each case or occurrence of a “thing” must be uniquely identified. Select one or
more attributes in each entity so that the values of these attributes may be
used to uniquely identify an occurrence of an entity. Select SocSecNumber as
the identifier or key for EMPLOYEE.
“Things” have associations among them. Derive relationships among entities
from the associations. CUSTOMER places ORDER. Derive a relationship
between the two entities from this association.
“Things” and associations conform to rules. One customer may place one or
more orders. There may some customers who have not placed any orders.
Rules govern the association between CUSTOMER and ORDER. From such
rules, derive the cardinality and optionality of the relationship.
Some “things” are subsets of other “things.” Individuals and institutions may be
customers to the organization. From this concept, derive INDIVIDUAL and
INSTITUTION as subsets of CUSTOMER.
In earlier versions of E-R modeling, the natural sequence had stopped with the
component “Cardinality/Optionality” mentioned above. Deeper observation of
real-world information requirements reveals the existence of supersets and subsets
as special types of entities. Therefore, the E-R data model was enhanced later to
include generalization and specialization.
Major Components
The components of the E-R model are practically the same as the object-based data
model. Both being techniques for building a semantic data model, you would almost
expect the building blocks to be the same. Therefore, as we describe the compo-
nents here or below in the chapter, you will note the special features of the E-R
model. Over the years, more precise notations have been included to indicate special
aspects of real-world information requirements.
Figure 7-2 presents the graphical notations or symbols for the various
components.
We mentioned above that E-R data model provides representations for differ-
ent variations within a component. For example, real-world information contains
variations in attributes. Sometimes, an attribute may be a composite of other attrib-
utes. Mostly, an attribute has a single value for one occurrence of an entity. However,
you also come across an attribute having multiple values for one occurrence of an
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