Primary keys allow entity sets and relationship sets to be expressed uniformly as relations

E/R diagram collection of tables For each entity set and each relationship

set there is a unique table

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Entity set: becomes a relation with the attributes of the entity set

Relationship: becomes a relation with attributes:◦ Key of connected entity sets◦ Attributes of the relationship

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Example E/R Diagram

Dog

Name Breed

Age

License #

Phone

Weight

Name

Name

Phone

Address

Owns

Owner

Kennel

Pays

Boards

Amount

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Entity Set to Relation

Dog

Name Breed

Age

WeightDog(Name,Breed,Age,Weight)

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Entity Set to Relation

OwnerKennel Pays

AmountNameName

Address

Pays(Kennel. Name,Owner.Name,Amount,Date)

Phone

Date

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Combining Relations: Many-One

OwnerKennel Pays

AmountNameName

Address

Combine relation for entity of “many” side (Owner) with relationship set, i.e., Owner’s-favorite(Name,Phone,FavoriteKennel.Name)

Favorite

Phone

Date

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Combining Relations: Many-Many

OwnerKennel Pays

NameName

Address

Combining Owner with pays:Owner(Kennel.Name,Owner.Name,Phone)

Phone

Kennel Name

Owner Name

Owner Phone

White Oak Mary 890-4421

Little Creek Mary 890-4421

White Oak Viki 987-4456

White Oak Joe 334-2234

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Redundancy

Relation for a weak entity set must include attributes for its complete key (including those that belong to other entities) as well as its own, non-key attributes

Supporting relationship is redundant and does not yield a relation

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Dog

Name Breed

Age OwnerOwns

Name

Phone

Owner(Owner.Name, Phone)Dog(Dog.Name,Owner.Name, Age,Breed)Owns(Dog.Name,Owner.Name1,Owner.Name2)

Must be the sameAlready in DogRedundant!

Three approaches:1. Object-oriented: each entity belongs to exactly one

class. Create a relation for each class with all its attributes.

2. E/R style: create one relation for each subclass with only the key attributes and the attributes of the subclass. Entity is represented in all relations to whose subclass entity set it belongs.

3. Null values: create on relation with all attributes. Entities have null values in attributes that do not belong to them.

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Example Subclass

Dog

Name Breed

Show-DogRank

ISA

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Object-Oriented Approach

Name Breed

Buddy Mix

Name Breed Rank

Pepper Germanshepherd 1st

Dog

Show dog

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Name Breed

Pepper Germanshepherd

Buddy Mix

Name Rank

Pepper 1st

Dog

Show dog

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Null Values

Name Breed Rank

Buddy Mix NULL

Pepper Germanshepherd 1st

Dog

Object-Oriented: good for queries like “find all G.S. dogs that ranked 2nd or above”.

E/R: good for “find all G.S. dogs (regardless of being a show dog or not)

Null values: may save space, but not good if too many attributes with null values.

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