L16 - ER Modeling 2

L16 - ER Modeling 2 - Entity Relationship Model Entity...

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Unformatted text preview: Entity Relationship Model Entity Relationship Model ¡ A database can be modeled as: ¢ a collection of entities , together with ¢ relationships among entities. ¡ An entity is an object that exists and is distinguishable from other objects. ¡ Entities have attributes ¢ these are properties that describe the entity and can be used to distinguish one from another ¡ An entity set is a set of entities of the same type that share the same properties. Relationship Sets Relationship Sets ¡ A relationship is an association among several entities and a relationship set is a mathematical relation among n ≥ 2 entities, each taken from entity sets {( e 1 , e 2 , … e n ) | e 1 ∈ E 1 , e 2 ∈ E 2 , …, e n ∈ E n } where ( e 1 , e 2 , …, e n ) is a relationship ¡ The degree of a relationship set refers to number of entity sets that participate in a relationship set. ¢ Relationship sets that involve two entity sets are binary (or degree two). Generally, most relationship sets in a database system are binary. ¢ Relationships between more than two entity sets are rare. Most relationships are binary. Attributes Attributes ¡ An entity is represented by a set of attributes , that is descriptive properties possessed by all members of an entity set ¡ The domain of an attribute is the set of permitted values for that attribute ¡ Attribute can be characterized along several dimensions: ¢ Simple vs. composite attributes. ¢ Single-valued vs. multi-valued attributes ¢ Derived attributes – those that can be computed from other attributes Mapping Cardinality Constraints for Relationships Mapping Cardinality Constraints for Relationships ¡ Mapping cardinality constraints express the number of entities to which another entity can be associated via a relationship set. ¢ They are most useful in describing binary relationship sets. ¡ For a binary relationship set the mapping cardinality must be one of the following types: ¢ One to one ¢ One to many ¢ Many to one ¢ Many to many Keys Keys ¡ A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity. ¡ A candidate key of an entity set is a minimal super key ¢ minimal in the sense that if we remove an attribute from the superkey it is no longer a superkey ¡ Although several candidate keys may exist, one of the candidate keys is selected to be the primary key . Relationship Sets and Keys Relationship Sets and Keys ¡ When we think in terms of representing a relationship set in the context of a relational database schema, we need to consider the concept of a key for a relationship set ¢ The combination of primary keys of the participating entity sets can be used to form a super key for a relationship set....
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This note was uploaded on 04/08/2008 for the course EE 468 taught by Professor Conry during the Spring '08 term at Clarkson University .

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L16 - ER Modeling 2 - Entity Relationship Model Entity...

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