The Entity-Relationship (ER) Model
1 Background
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Used for conceptual database design
Relies on concepts of entities, attributes, and relationships
Benefits from pictorial representation, called ER diagrams
2 An Entity-Relationship (ER) Diagram
3 Entity Types
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Entity types represent sets of objects and are pictured by rectangular nodes
.
An entity set is the collection of instances (i.e., entities) represented by an entity
type.
An entity set is also called an extension of the entity type
4 Relationship Types
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Relationship types associate entity types. They are pictured by Diamond nodes,
and edges connecting to the related entity types.
A relationship set is the collection of instances (i.e., relationships between
objects) represented by a relationship type.
Relationship types may associate an entity type with itself. In such a case, the
roles of the entity types in the relationship type are listed on the edges, and the
relationship is said to be recursive.
5 Attributes
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Entity types and relationship types might have attributes.
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The value set, or domain, of an attribute is the set of values that may be assigned
to the attribute. Mathematically,
attribute : entity
power-set(domain)
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Atomic attribute types, pictured by oval nodes
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Composite attribute types, achieved by concatenating simpler attribute types,
pictured by trees of atomic attributes
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Multivalued attribute types
A ‘blue and red’ shirt
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Derived attribute types displayed in dashed ovals
A ‘age’ from ‘birth date’
6 Keys and Weak Entity Types
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Key attribute types, identify the instances, may be consisted of more than one
attribute, displayed with underlined attribute type names
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Weak entity types have no keys. Displayed by double-rectangular nodes
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To be identified, instances of weak entity type require an identifying relationship
type that relates them to an identifying entity type. Such relations are displayed
by double-diamond nodes
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Weak entity types typically have partial key for distinguishing their instances.
Regular entity types with keys are sometimes called strong entity types.
7 Structural Relationship Constraints
Cardinality ratio constraint
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Specifies the number of relationship instances an entity can participate in
Displayed on the diamonds
Participation constraint (minimum cardinality constraint)
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specifies whether the existence of an entity depends on its being related to
another entity via the relationship type
Total participation constraints require the participation of every entity the
relationship (displayed by double line). Also called existence dependency.
Partial participation constraints (displayed by a single line).
Cardinality constraint
Specifies lower and upper bounds on the number of relationships each entity can
participate in.
8 Domain Analysis
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Entity set + relationship set + constraints
To see if the information kept in database is exactly what the user wants.
Summary of Notation
Entity
Weak Entity
Relationship
Identifying Relationship
Attribute
Key Attribute
Multivalued Attribute
Composite Attribute
Derived Attribute
Total participation of E2 in R
Cardinality ratio 1 : N for E1 : E2 in R
Structural constraint (min,max) on participation of E in
R
Reference: Ch. 3 in textbook.