1. Transformation of E/R Diagram to Relation

2. Transformation of E/R Diagram to Relation
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
CSCE 520
11/16/2018

3. Transformation from E/R to Relation
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
CSCE 520
11/16/2018

4. Example E/R Diagram Dog Kennel Owner Breed Name Name License # Boards
Age
Address
Owns
Phone
Weight
Pays
Owner
Amount
Name
Phone
CSCE 520
11/16/2018

5. Entity Set to Relation Dog Dog(Name,Breed,Age,Weight) Breed Name Age
CSCE 520
11/16/2018

6. Entity Set to Relation Kennel Owner
Name
Amount
Date
Name
Address
Kennel
Pays
Owner
Phone
Pays(Kennel. Name,Owner.Name,Amount,Date)
CSCE 520
11/16/2018

7. Combining Relations: Many-One
Name
Amount
Date
Name
Address
Kennel
Pays
Owner
Phone
Favorite
Combine relation for entity of “many” side (Owner) with relationship set, i.e.,
Owner’s-favorite(Name,Phone,FavoriteKennel.Name)
CSCE 520
11/16/2018

8. Combining Relations: Many-Many
Name
Name
Address
Kennel
Pays
Owner
Phone
Combining Owner with pays:
Owner(Kennel.Name,Owner.Name,Phone)
CSCE 520
11/16/2018

9. Many-many Relationship
Kennel Name
Owner Name
Owner Phone
White Oak
Mary
Little Creek
Viki
Joe
Redundancy
11/16/2018
CSCE 520

10. Weak Entity Sets to Relation
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
CSCE 520
11/16/2018

11. Example: Weak Entity Set
Breed
Name
Name
Dog
Owns
Owner
Phone
Age
Owner(Owner.Name, Phone)
Dog(Dog.Name,Owner.Name, Age,Breed)
Owns(Dog.Name,Owner.Name1,Owner.Name2)
Already in Dog
Redundant!
Must be the same
CSCE 520
11/16/2018

12. Entity Sets with Subclasses
Three approaches:
Object-oriented: each entity belongs to exactly one class. Create a relation for each class with all its attributes.
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.
Null values: create on relation with all attributes. Entities have null values in attributes that do not belong to them.
CSCE 520
11/16/2018

13. Example Subclass
Name
Breed
Dog
ISA
Show-Dog
Rank
CSCE 520
11/16/2018

14. Object-Oriented Approach
Name
Breed
Buddy
Mix
Dog
Name
Breed
Rank
Pepper
Germanshepherd
1st
Show dog
CSCE 520
11/16/2018

15. E/R Approach Name Breed Pepper Germanshepherd Buddy Mix Name Rank
Dog
Name
Rank
Pepper
1st
Show dog
CSCE 520
11/16/2018

16. Null Values Name Breed Rank Buddy Mix NULL Pepper Germanshepherd 1st
Dog
CSCE 520
11/16/2018

17. Comparisons
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.
CSCE 520
11/16/2018