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Entity-Relationship Model
A conceptual model of data.
Provides a series of constructs for describing the data requirements of an application.
Entities
Represent classes of objects that have properties in common - Students
In a schema every entity has a unique name
Relationships
represent logical links between two or more entities - Exam is a relation between student and subject
An occurance of a relationship is an tuple made up of occurances of entities.
Each relationship has a unique name
Attributes
Describe the elementary properties of entities - Sudent has a number, a name and a uid
Atributes have values
domain of an attribute are the permitted values - e.g the domain of a dice is 1..6
Cardinality
Specify maximum and minimum number of relationship occurances in which an instance of an entity can participate
Minimum = 0 - paticipation is optional
Minimum = 1 - participation is mandatory
Maximum = 1 - instance of entity is associated with only 1 occurance of relationship
Maximum = n - entity instance can be involved in an arbitrary number
of relationships
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Page updated August 25, 2004
