ENTITYRELATIONSHIP ER MODEL

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ENTITY-RELATIONSHIP (E/R) MODEL
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Reading and Exercises

• Database Systems- The Complete Book Chapter 2
• Self testing exercises
• 2.1.3 and 2.1.4
• 2.2.4
• 2.3.1.b
• Any of the remaining exercises in the textbook
• Following lecture slides are modified from Jeff
  Ullmans slides for Fall 2002 -- Stanford

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Purpose

• Informal Database Design
• Entity-relationship diagram
• Easy to convert to implementations (e.g.,
  relational databases)

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Entity Sets

• Database collection of entities and relationship
  among entities
  
• Entity object that exists and distinguishable
  from other objects
  
• Entity set collection of similar objects
• Attribute property of an entity set
• Each entity in the set has the same properties

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Attributes

• Domain set of permitted values for each
  attributes
  
• Attribute types
• Simple v.s. composite
• Single-valued v.s. multi-valued
• Derived

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E/R Diagram

• Entity sets diagrams
• Attributes oval
• Relationship sets between entity sets diamond

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Example E/R Diagram
Breed
Name
Name
License
Dog
Kennel
Boards
Age
Address
Phone
Owns
Weight
Pays
Owner
Phone
Name
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Relationship Set

• Current value of an entity set is the set of
  entities that belong to it
  
• Value of a relationship is a set of lists of
  currently related entities (one from each entity
  sets)

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Example

• Example of the Owns relation

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Degree of Relationship Sets

• Number of entity sets participating in a
  relationship set
  
• Binary relationship set two entity sets (most
  common)
  
• Multiway relationship set connects more than two
  entity sets
  
• E.g., An owner frequents certain kennels for
  certain dogs
  
• Binary relationship cant represent these
  requirements
  
• Need 3-way relationship

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Example 3-Way Diagram
Breed
Name
Name
License
Dog
Kennel
Age
Address
Must Board
Phone
Weight
Owner
Phone
Name
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Example 3-Way Relation

• Example of the Must-Board relation

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Mapping Cardinality

• Number of entities to which another entity can be
  associated via a relationship set
  
• One-one
• Many-one (One-many)
• Many-many

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One-One Relationship

• Each entity of either entity set is related to at
  most one entity of the other set
  
• E.g., assume each owner is allowed to have only 1
  dog, and each dog must belong to 1 owner. The
  Owns relation between Dog and Owner is one-one

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Many-One Relationship

• Each entity of the first set is connected to at
  most one entity in the second set
  
• Entities in the second set can connect to 0, 1,
  2, or more entities of the first set.
  
• E.g., assume, each dog has at most 1 owner, but
  an owner may have any number of dogs. TheOwns
  relation from Dog to Owner is is many-one.

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Many-Many Relationship

• Entity of either set can be connected to many
  entities of the other set
  
• E.g., assume that a dog may belong to several
  persons, and a person may have any number of
  dogs. The Own relation between Dog and Owner is
  many-many

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Cardinality of Relationships
one-one many-one many-many
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Representing Cardinality

• One-one arrow in each side of the relationship
• Many-one arrow on the one side
• Exactly one each entity of the one set must be
  related to exactly on entity of the other set.
  Rounded arrow on the one side

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Relationship Attribute

• Relationships may have attributes
• Property of the relationship
• E.g., amount the owners pays to the kennel for
  boarding their dog

Amount
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Relationship Attribute

• Other representation Create an entity set to
  represent relationship property
  
• Create 3-way relationship among all entities

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Relationship Attributes
Pays
Owner
Kennel
Cost
Amount
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Roles

• Entity set may appear more then once in a
  relationship
  
• Label the edges between the relationships and the
  entity set with names called roles.
  
• E.g., relationships among the dogs

Related to
ancestor
Dog
descendant
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Subclasses in E/R

• Special case, fewer entities, more properties
• E.g., show dog is a dog, but not all dogs are
  show dogs. It also have properties, type of
  competition, rank, etc.
  
• Assume subclasses form a tree (no multiple
  inheritance)
  
• ISA relationship

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Example Subclass
Name
Age
Breed
Weight
Dog
ISA
Type
Show-Dog
Rank
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Keys

• Set of attributes for one entity set such that no
  two entities in the set has the same value for
  all the attributes of the key
  
• Each entity set must have a key

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Keys

• Super key set of one or more attributes whose
  value iniquely determine each entity
  
• Candidate key minimal super key
• Primary key a selected candidate key

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Keys in E/R Diagram

• Underline the key attributes
• In ISA hierarchy only the root entity set has a
  key, it is also a key for all subclasses

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Example Key for Owner
Owner
Phone
Name
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Example Multi-Attribute Key for Owner
There are more than One person with the Same na
me, but they
Live at different Places.
Owner
Name
Address
Phone
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Weak Entity Sets

• Entities of an entity set need help to identify
  them uniquely
  
• Entity set E is said to be weak if in order to
  identify entities of E uniquely, we need to
  follow one of more many-one relationships from E
  and include the key of the related entities from
  the connected entity sets.

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Example Weak Entity Set
Breed
Name and breed do not uniquely
Identify a dog. However, name and breed and The
name of the owner will Uniquely identify a dog.
Representation double line Around
relationship diamond.
Name
Dog
Owns
Owner
Many-one relationship
Phone
Name
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Keys for Relationship Sets

• Combination of primary keys of participating
  entity sets
  
• Must consider mapping cardinalities to choose the
  candidate keys
  
• Need semantics of relationship set for selecting
  primary key

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Design Techniques

• Avoid redundancy
• Limit use of weak entity sets
• Dont use entity set when can use attribute
• Binary v.s. multiway relationship sets

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Avoid Redundancy

• Repeat the same thing in two or more different
  ways
  
• Bad because
• Wastes space
• Encourages inconsistency

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Good Design Avoid Redundancy
Breed
Name
Name of the owner Listed only once For each dog
Dog
Age
Owns
Weight
Owner
Phone
Name
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Bad Design Avoid Redundancy
Breed
Name
Name of the owner Listed twice, once for each
dog and once
For the relationship Owns
Owner
Dog
Age
Owns
Weight
Owner
Phone
Name
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Entity Set v.s. Attribute
Good
Bad
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Weak Entity Set

• Try to create unique ids for entity sets
• Use weak entity sets only if impossible to create
  unique ids
  
• E.g., can not require all dog owners to give
  unique names for their pets