In 1976 ,Chen developed the Entity-Relationship Diagrams ,a high-level data model that is useful in developing a conceptual design for database . An ER diagram is a diagram containing entities or “items”, relationships among them, and attributes of the entities .
The E-R model is one of the best known tools for logical database design. Within the database community, it is considered a natural and easy-to-understand way of conceptualizing the structure of database. Claims that have been made for it include the following: it is simple and easily understood by non-specialist ,it is easily conceptualized ,the basic constructs (entities and relationships) are highly intuitive and thus provide a natural way of representing a user’s information requirements , and it is a model that describes a world in terms of entities and attributes that is most suitable for computer naïve end users.
In E-R diagram the emphasis is on representing the schema a rather than the instances. This is more useful in database design because a database schema changes rarely ,whereas the contents of the entity sets changes frequently. In addition ,the schema is usually easier to display that the extension of database ,because it is much smaller
Entity-relationship diagrams were first proposed as a means of quickly obtaining, with minimum effort, a good sense of the structure of a database. They are used to plan and design a database and to model a systems data.
Entities:- * An entity represents the principle data objects about which information is to be collected. * Collective nouns, or nouns, are usually used to name (describe) entities * For example, each person in an enterprise is an entity. * An entity has a set of properties, and the values for some set of properties may uniquely identify an entity. For instance, a person may have a