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.
* 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 person_id property whose value uniquely identifies that person.
* An entity set is a set of entities of the same type that share the same properties, or attributes. The set of all persons who are customers at a given bank, for example, can be defined as the entity set customer.
* An attribute is one of the various properties that describe the entity’s characteristics. These properties usually present a single fact – they are atomic. *
The designation of an attribute for an entity set expresses that the database stores similar information concerning each entity in the entity set; however, each entity may have its own value for each attribute. * Possible attributes of the customer entity set are customer-id, customer-name, customer-street, and customer-city.
Domain (value set):-
* For each attribute, there is a set of permitted values, called the domain, or value set, of that attribute.
* The domain of attribute customer-name might be the set of all text strings of a certain length.
Types of attribute in the E-R model:-
The attributes used in the ER model can be categorized as
1. Simple or Composite
2. Single Valued or Multi Valued
3. Stored or Derived.
1. Simple or Composite
* The attribute which are not divided into subparts are called simple attributes.
* For example, an attribute customer-id is a simple attribute.
* Composite attributes, on the other hand, can be divided into subparts (that is, other attributes).
For example, an attribute name could be structured as a composite attribute consisting of first-name, middle-initial, and last- name. *
Using composite attributes in a design schema is a good choice if a user will wish to refer to an entire attribute on some occasions and to only a component of the attribute on other occasions. Suppose we were to substitute for the customer entity-set attributes customer-street and customer-city the...
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