Transformation of E/R Diagram to Relation

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

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

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

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

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

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

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

Many-many Relationship Kennel Name Owner Name Owner Phone White Oak Mary 890-4421 Little Creek Viki 987-4456 Joe 334-2234 Redundancy 11/16/2018 CSCE 520

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

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

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

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

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

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

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

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