Lecture 02 - Lecture 02 Comp355 E-R Model Constructs Entity...

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Lecture 02 Comp355
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E-R Model Constructs Entity instance - person, place, object, event, concept (often corresponds to a row in a table) Entity Type – collection of entities (often corresponds to a table) Attribute - property or characteristic of an entity type (often corresponds to a field in a table) Relationship instance – link between entities (corresponds to primary key-foreign key equivalencies in related tables) Relationship type – category of relationship…link between entity types
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Sample E-R Diagram (figure 3-1)
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Figure 3-2 -- Basic E-R Notation Entity symbols Relationship symbols Attribute symbols A special entity that is also a relationship
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What Should an Entity Be? SHOULD BE: An object that will have many instances in the database An object that will be composed of multiple attributes An object that we are trying to model SHOULD NOT BE: A user of the database system An output of the database system (e.g. a report)
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Inappropriate entities System user System user System output System output Appropriate entities Figure 3-4
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Attributes Attribute - property or characteristic of an entity type Classifications of attributes: Simple versus Composite Attribute Single-Valued versus Multivalued Attribute Stored versus Derived Attributes Identifier Attributes
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Identifiers (Keys) Identifier (Key) - An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type Simple Key versus Composite Key Candidate Key – an attribute that could be a key…satisfies the requirements for being a key
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Characteristics of Identifiers Will not change in value Will not be null
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Lecture 02 - Lecture 02 Comp355 E-R Model Constructs Entity...

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