1 / 22

Relational Databases

Introduction to the relational data model, conversion from entity-relationship model, combining relations, weak entity sets, and representing subclasses in relational databases.

vhintz
Télécharger la présentation

Relational Databases

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Relational Databases CPSC 315 – Programming Studio Spring 2012 Project 1, Lecture 2 Slides adapted from those used by Jeffrey Ullman, via Jennifer Welch

  2. Relational Data Model • Relations are stored in tables • e.g. Sponsor(Senator,Bill) Atrributes Tuples

  3. Schemas • A relation schema is a relation name and a list of attributes • Sponsor(Senator,Bill) • A database is a collection of relations • A database schema is the set of all the relation schemas in the database

  4. Converting from Entity-Relationship Model • ER: Entity set -> relation • ER Attributes become Relational attributes • ER: Relationship -> relation • Keys of connected ER entity sets become Relational attributes

  5. ER Entity Sets Name Party Organization Senator Lobbyist State Name Years Bill Name Text

  6. Relations • Senator(Name,Party,State,Years) • Bill(Name,Text) • Lobbyist(Name,Organization)

  7. ER Relationships Name Contributed Party Organization Senator Lobbyist State Sponsored Name Years Wrote Bill Name Text

  8. Relations • Sponsored(Senator,Bill) • Wrote(Bill,Lobbyist) • Contributed(Senator,Lobbyist) • Remember, each of these is expressed as a table (with the columns given by the “parameters”) • Notice that columns can refer to “bigger” items, with even more attributes

  9. Combining Relations • Relations can sometimes be combined. • Assume a “base” entity set with its relation. • If there is a many-to-one relation, then it can be combined with the base entity set. • Should not combine many-to-many • Would cause redundancy – each of the many stored

  10. Combining Relations • Example (many-to-one): (Good) • Person(Name, Birthdate, Height, Weight, Eye Color, Hair Color) • BornIn(Person,Town) • Person(Name, Birthdate, Height, Weight, Eye Color, Hair Color, Town) • Example(many-to-many): (Bad) • Senator(Name, Party, State, Years) • Sponsored(Senator, Bill) • Senator(Name, Party, State, Years, Bill)

  11. Weak Entity Sets • The relation for a weak entity set must contain all the elements of its key • Supporting relationships are usually redundant (unless possibly multi-way)

  12. Weak Entity Set Example First Name Last Name Number Note arrrow: indicates many to one. Baseball Player Position Birthdate Plays On Team Nationality Salary Name City

  13. Weak Entity Set Example • Team(Name, City) • Baseball Player(Number, TeamName, First Name, Last Name, Position, Birthdate, Nationality, Salary)

  14. Weak Entity Set Example • Team(Name, City) • Baseball Player(Number, TeamName, First Name, Last Name, Position, Birthdate, Nationality, Salary) • Note that we don’t need PlaysOn(BaseballPlayer.Number, BaseballPlayer.TeamName, Team.Name)

  15. Weak Entity Set Example • Team(Name, City) • Baseball Player(Number, TeamName, First Name, Last Name, Position, Birthdate, Nationality, Salary) • Note that we don’t need PlaysOn(BaseballPlayer.Number, BaseballPlayer.TeamName, Team.Name) Redundant (same)

  16. Weak Entity Set Example • Team(Name, City) • Baseball Player(Number, TeamName, First Name, Last Name, Position, Birthdate, Nationality, Salary) • Note that we don’t need PlaysOn(BaseballPlayer.Number, BaseballPlayer.Team.Name)

  17. Weak Entity Set Example • Team(Name, City) • Baseball Player(Number, TeamName, First Name, Last Name, Position, Birthdate, Nationality, Salary) • Note that we don’t need PlaysOn(BaseballPlayer.Number, BaseballPlayer.Team.Name) Already Included

  18. SubclassesDifferent Options • Different ways to represent subclasses Name Elected Official Party isa U.S. Representative District

  19. Object-Oriented Style • One relation for each subset, including all “inherited” attributes

  20. Entity-Relationship Style • One relation for each subclass (including key)

  21. Using Nulls Style • One relation total, with nulls for unknown information • Can save space, but problematic if multiple subclasses or lots of NULLs

  22. Keys • A Key “functionally determines” all other attributes of the relation • Given a relation and a key, there is only one tuple that corresponds to it • There are subtle differences from an E-R key, which we won’t go into.

More Related