Saturday, 11 February 2012

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".


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.

Physical and Abstract Entity
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.


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.
Entity and its attributes
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.
Employee entity and its attribute values
Fig 7.8 - Employee entity and its attribute values

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