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Analysis and Design
An entity set is a set of entities of the same type that share the same properties,
or attributes. For example, all software engineers working in the department involved
in the Internet projects can be defined as the entity set InternetGroup. The individual
entities that constitute a set are called extension of the entity set. Thus, all
individual software engineers of in the Internet projects are the extensions of
the entity set InternetGroup.
Entity sets don’t need to be disjointed. For example, we can define an
entity set Employee. An employee may or may not be working on some Internet projects.
In InternetGroup we will have some entries that are there in Employee entity set.
Therefore, entity sets Employee and InternetGroup are not disjoint.
A database usually contains groups of entities that are similar. For example,
employees of a company share the same attributes. However, every employee entity
has its own values for each attribute. An entity type defines a set of entities
that have same attributes. A name and a list of attributes describe each entity
Fig. 7.10 shows two entity types Employee and Product. Their attribute list
is also shown. A few members of each entity type are shown.
Fig. 7.10 Two entity types and some of the member entities of each
An entity type is represented in ER diagrams as rectangular box and the corresponding
attributes are shown in ovals attached to the entity type by straight lines. See
fig 7.7. in the section E R Model Concept
An entity type is basically the schema or intension or structure for the set
of entities that share the same structure whereas the individual entities of a
particular entity type are collectively called entity set. The entity set is also
called the extension of the entity type.
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