Database System Concepts Modeling Concepts Entity- Relationship Diagrams Extended E-R Features E-R Design Database System Concepts Chapter 6: Entity-Relationship Model Departamento de Engenharia Inform´ atica Instituto Superior T´ ecnico 1 st Semester 2008/2009 Slides (fortemente) baseados nos slides oficiais do livro “Database System Concepts” c Silberschatz, Korth and Sudarshan.
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Database System Concepts - Chapter 6: Entity-Relationship Model
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DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Database System ConceptsChapter 6: Entity-Relationship Model
Departamento de Engenharia InformaticaInstituto Superior Tecnico
An attribute can also be property of a relationship set.For instance, the depositor relationship set between entitysets customer and account may have the attributeaccess-date
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Degree of a Relationship Set
Refers to number of entity sets that participate in arelationship set.
Relationship sets that involve two entity sets are binary (ordegree two).
Relationship sets may involve more than two entity sets.
Example: Suppose employees of a bank may have jobs(responsibilities) at multiple branches, with different jobsat different branches. Then there is a ternary relationshipset between entity sets employee, job, and branch
Relationships between more than two entity sets are rare.Most relationships are binary.
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Attributes
An entity is represented by a set of attributes, that isdescriptive properties possessed by all members of anentity set.
Example:
customer = (customer_id, customer_name,
customer_street, customer_city)
loan = (loan_number, amount)
Domain - the set of permitted values for each attribute
Attribute types:
Simple and composite attributes.Single-valued and multi-valued attributes
Example: multivalued attribute: phone numbers
Derived attributes
Can be computed from other attributes
Example: age, given date of birth
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Mapping Cardinality Constraints
Express the number of entities to which another entity canbe associated via a relationship set.
Most useful in describing binary relationship sets.
For a binary relationship set the mapping cardinality mustbe one of the following types:
One to oneOne to manyMany to oneMany to many
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Mapping Cardinalities
One to one One to many
Note: Some elements in A and B may not be mapped to anyelements in the other set
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Mapping Cardinalities
Many to one Many to many
Note: Some elements in A and B may not be mapped to anyelements in the other set
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Keys
A super key of an entity set is a set of one or moreattributes whose values uniquely determine each entity.
A candidate key of an entity set is a minimal super key
Customer id is candidate key of customeraccount number is candidate key of account
Although several candidate keys may exist, one of thecandidate keys is selected to be the primary key.
The combination of primary keys of the participatingentity sets forms a super key of a relationship set.
(customer id, account number) is the super key ofdepositorMust consider the mapping cardinality of the relationshipset when deciding the what are the candidate keys
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Outline
1 Modeling Concepts
2 Entity-Relationship Diagrams
3 Extended E-R Features
4 E-R Design
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
E-R Diagrams
Rectangles represent entity sets.
Diamonds represent relationship sets.
Lines link attributes to entity sets and entity sets torelationship sets.
E-R Diagram With Composite, Multivalued, andDerived Attributes
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Relationship Sets with Attributes
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Roles
Entity sets of a relationship need not be distinct
The labels “manager” and “worker” are called roles; theyspecify how employee entities interact via the works for
relationship set.
Roles are indicated in E-R diagrams by labeling the lines thatconnect diamonds to rectangles.
Role labels are optional, and are used to clarify semantics of therelationship
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Cardinality Constraints
We express cardinality constraints by drawing either adirected line (→), signifying “one”, or an undirected line(−), signifying “many”, between the relationship set andthe entity set.
many-to-many relationship
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Cardinality Constraints
We express cardinality constraints by drawing either adirected line (→), signifying “one”, or an undirected line(−), signifying “many”, between the relationship set andthe entity set.
one-to-many relationship
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Cardinality Constraints
We express cardinality constraints by drawing either adirected line (→), signifying “one”, or an undirected line(−), signifying “many”, between the relationship set andthe entity set.
many-to-one relationship
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Participation of an Entity Set in a Relationship Set
Total participation (double line): every entity in the entityset participates in at least one relationship in therelationship set
E.g. participation of loan in borrower is total: every loanmust have a customer associated to it via borrower
Partial participation: some entities may not participate inany relationship in the relationship set
Example: participation of customer in borrower is partial
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
E-R Diagram with a Ternary Relationship
Cardinality Constraints:
We allow at most one arrow out of a ternary (or greaterdegree) relationship to indicate a cardinality constraintE.g. an arrow from works on to job indicates eachemployee works on at most one job at any branch.
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Weak Entity Set
An entity set that does not have a primary key is referredto as a weak entity set.
The existence of a weak entity set depends on theexistence of a identifying entity set
it must relate to the identifying entity set via a total,
one-to-many relationship set from the identifying to theweak entity setIdentifying relationship depicted using a double diamond
The discriminator (or partial key) of a weak entity set isthe set of attributes that distinguishes among all theentities of a weak entity set.
The primary key of a weak entity set is formed by theprimary key of the strong entity set on which the weakentity set is existence dependent, plus the weak entityset’s discriminator.
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Weak Entity Set (cont.)
We depict a weak entity set by double rectangles.
We underline the discriminator of a weak entity set with adashed line.
payment number - discriminator of the payment entity set
Primary key for payment - (loan number,
payment number)
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Outline
1 Modeling Concepts
2 Entity-Relationship Diagrams
3 Extended E-R Features
4 E-R Design
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Specialization/Generalization
Depicted by a triangle component labeled ISA (E.g. customer“is a” person).
Attribute inheritance - a lower-level entity set inherits all theattributes and relationship participation of the higher-levelentity set to which it is linked.
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Constraints on a Specialization/Generalization
Constraint on whether or not entities may belong to morethan one lower-level entity set within a singlegeneralization.
Disjoint - an entity can belong to only one lower-levelentity set
Noted in E-R diagram by writing disjoint next to the ISA
triangle
Overlapping - an entity can belong to more than onelower-level entity set
Completeness constraint specifies whether or not an entityin the higher-level entity set must belong to at least one ofthe lower-level entity sets within a generalization.
Total - an entity must belong to one of the lower-levelentity sets
Noted in E-R diagram by a double line over the ISA
triangle
Partial - an entity need not belong to one of the lower-levelentity sets
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Aggregation
Consider the ternary relationship works on, which we saw earlierSuppose we want to record managers for tasks performed by anemployee at a branch
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Outline
1 Modeling Concepts
2 Entity-Relationship Diagrams
3 Extended E-R Features
4 E-R Design
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Design Issues
The use of an attribute or entity set to represent an object.
Whether a real-world concept is best expressed by anentity set or a relationship set.
The use of a ternary relationship versus a pair of binaryrelationships.
The use of a strong or weak entity set.
The use of specialization/generalization - contributes tomodularity in the design.
The use of aggregation - can treat the aggregate entity setas a single unit without concern for the details of itsinternal structure.
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Mapping Cardinalities affect ER Design
Can make access-date an attribute of account, instead ofa relationship attribute, if each account can have only onecustomer
DatabaseSystem
Concepts
ModelingConcepts
Entity-RelationshipDiagrams
Extended E-RFeatures
E-R Design
Mapping Cardinalities affect ER Design
Can make access-date an attribute of account, instead ofa relationship attribute, if each account can have only onecustomer