Data Models in DBMS
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