An entity-relationship data model (E-R Model) is a detailed, logical representation of the data in a database. The E-R model is normally expressed as an entity relationship diagram, which is a graphical representation of an E-R model. The major constructs of E-R models are entities, relationships, and associated attributes.
Entities: a person, place, object, event, or concept in the user environment about which an organization wishes to maintain data. Each entity type in a an E-R model is given a name. Since the name represents a class (or set), it is singular. For the Professor database they represented by a rectangle and are:
Course - Student - Test - Note -- Clipping
Attribute: each entity type has a set of attributes associated with it. An attribute is a property or characteristic of an entity that is of interest to the organization, and is included in the database.
Every entity type must have an attribute or set of attributes that uniquely identifies each instance and clearly distinguishes that instance from other instances of the same type.
Candidate key: is an attribute (or combination of attributes) that uniquely identifies each instance of an entity type.
Some entities may have more than one candidate key. If there is more than one candidate key, the designer must choose one of the candidate keys as a primary key.
Primary key: A candidate key that has been selected as the identifier.
For the Professor database they are listed next to the entity, with the primary key underlined.
Relationships: are the glue that holds together the various components of an E-R model. It is an association between the instances of one or more entity types that is of interest to the organization.
Degree of a Relationship is the number of entity types that participate in that relationship. Unary, Binary or Ternary relationships. The Professor Database is binary.
Click here to go to the E-R Model for the Professor Database.
e-mail:jsulliva@raritanval.edu
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