that4loopgirl

The E – R data model employs three notations:  Entity sets,  Relationship sets, and  Attributes. 1.  Entity sets An entity is a thing or object in real world that is different from other objects. For example, each employee in a company is an entity. An entity has a set of properties, and unique value for some properties. For example, the employee may have employee_id property whose value uniquely identifies the employee. Thus, 0201 value for employee_id would uniquely identify the employee in a company. An entity set is a set of entities of the same type that may share same properties, or attributes. For example, the set of all persons who are employees at a company can be defined as the entity set employee. Entity sets do not need to be disjoint. For example, a person may be an employee entity, a customer entity, both or neither. An entity is represented by set of attributes . Attributes are properties possessed by each member of the entity set. Each entity may

Data model is a collection of conceptual tools for describing data, relationships of data, data semantics, and consistency constraints. There are two models to describe the design of the database at the logical level.  The entity – relationship model The entity – relationship (ER) data model consists of a collection of basic objects, called entities , and of relationships among these objects. Entities are described by set of attributes in the database. For example, the attributes employee_name and employee_salary may describe particular information in a company, of the employee entity set. An extra attribute employee_id is used to uniquely identify the employees.  A relationship is an association among several entities. The overall logical schema of the database can be represented graphically by an E – R diagram . It consists of following components: ·        Rectangles , to represent entity sets ·        Ellipses , to represent attributes ·        Diamonds , to represe