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Analysis and Design
E-R Model concept
The ER data modeling techniques is based on the perception of a real world
that consists of a set of basic objects called entities, and of relationships
among these objects. In ER modeling, data is described as entities, relationships,
and attributes. In the following section, entities and attributes are discussed.
Later, entity types, their key attributes, relationship types, their structural
constraints, and weak entity types are discussed. In the last, we will apply ER
modeling to our case study problem "Library management system".
Entities and Attributes
One of the basic components of ER model is entity. An entity is any distinguishable
object about which information is stored. These objects can be person, place,
thing, event or a concept. Entities contain descriptive information. Each entity
is distinct.
An entity may be physical or abstract. A person, a book, car, house, employee
etc. are all physical entities whereas a company, job, or a university course,
are abstract entities.
Fig 7.6 - Physical and Abstract Entity
Another classification of entities can be independent or dependent (strong
or weak) entity.
ntities are classified as independent or dependent (in some methodologies,
the terms used are strong and weak, respectively). An independent entity is one,
which does not rely on another entity for identification. A dependent entity is
one that relies on another entity for identification. An independent entity exists
on its own whereas dependent entity exists on the existence of some other entity.
For example take an organization scenario. Here department is independent entity.
Department manager is a dependent entity. It exists for existing depts. There
won't be any department manager for which there is no dept.
Some entity types may not have any key attributes of their own. These are called
weak entity types. Entities belonging to a weak entity type are identified by
being related to specific entities from another entity type in combination with
some of their attribute values. For example, take the license entity. It can't
exist unless it is related to a person entity.
Attributes
After you identify an entity, then you describe it in real terms, or through
its attributes. Attributes are basically properties of entity. We can use attributes
for identifying and expressing entities. For example, Dept entity can have DeptName,
DeptId, and DeptManager as its attributes. A car entity can have modelno, brandname,
and color as its attributes.
A particular instance of an attribute is a value. For example, "Bhaskar"
is one value of the attribute Name. Employee number 8005 uniquely identifies an
employee in a company.
The value of one or more attributes can uniquely identify an entity.
Fig 7.7 - Entity and its attributes
In the above figure, employee is the entity. EmpNo, Name, Designation and Department
are its attributes.
An entity set may have several attributes. Formally each entity can be described
by set of <attribute, data value> pairs.
Fig 7.8 - Employee entity and its attribute values
See Also
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