1. Copyright © 2003 Addison-Wesley Sree Nilakanta

2. Copyright © 2003 Addison-Wesley Developing Relational Models What is the relational model and what is a relational database? How are relational schemas and their keys defined? How are entity classes and entities represented in a relational model and database? How are attributes and attribute values represented in a relational model and database? How are relationship types and relational models represented in a relational model and database? How are weak entity classes represented in a relational model? How is inheritance represented in a relational model?

3. Copyright © 2003 Addison-Wesley Basics of the Relational Model The relational model represents information in tables (called relations) –Each table represents a set of entities –Each column of a table represents the attribute values of the entities –Each row of a table represents a single entity A database schema is a collection of table definitions (relation schemas) A relational database is a collection of tables –Each table stores objects for a single relation schema

4. Copyright © 2003 Addison-Wesley Relation Schemas and Keys The rows of a relational table are unique –No 2 rows have the same values for all of the attributes A key is a collection of attributes in which –No 2 rows have the same values for all attributes in the key Every table must have a key –Why? A relation schema is the specification of the structure of a table –Name of the table –Name and type of each attribute –Declaration of the key A key declaration is a constraint –A table is not allowed to have 2 different rows that have the same value for the key –Database systems enforce key constraints By blocking any attempt to modify a table that will result in a violation of the key constraint

5. Copyright © 2003 Addison-Wesley Relation is not Relationship Be careful of these two words –Relation –Relationship A relation is a table that contains a set of entities A relationship is a connection between two entities We must be very careful to use the correct word –Listen closely to the lectures and correct me if I get it wrong!

6. Copyright © 2003 Addison-Wesley Translating E-R Diagrams We create a relational model from an E-R model –For each component of the E-R diagram, create a representation in the relational model This chapter describes how to systematically translate an E-R model into a database schema –Entity classes –Simple attributes –Composite attributes –Key attributes –Relationship types of different cardinalities –Weak entity classes –Multi-valued attributes –Inheritance

7. Copyright © 2003 Addison-Wesley Representing Entity Classes For each strong entity classes in your E-R model create a relation schema: Rule 1a: Define a relation schema by the same name. Rule 1b: For each single-valued attribute of the entity class –create an attribute by the same name in the relation schema and specify a type for the attribute Rule 1c: Define the key of the new relation schema as the key of the entity class –If the entity class key consists of multiple simple attributes, the key of the relation schema will be that set of attributes. –Underline your selected key attribute in each schema in order to identify the key.

8. Copyright © 2003 Addison-Wesley Composite Attributes Rule 2. For each composite attribute of a strong entity class –create an attribute in the relation schema for each component attribute –If appropriate, use the name of the composite attribute as a prefix for each of the component attribute names Schema: Customer (accountId string, lastName string, firstName string, street string, city string, state string, zipcode string, balance number) accountIdlastNamefirstNamestreetcitystatezipcodebalance 101BlockJane1010 Main St.ApopkaFL304580.00 102HamiltonCherry3230 Dade St.Dade CityFL305554.47 103HarrisonKate103 Dodd HallApopkaFL3045730.57 104BreauxCarroll76 Main St.ApopkaFL3045834.58

9. Copyright © 2003 Addison-Wesley Representing Relationship Types as Attributes A relationship type may be represented by attributes in a relation schema Add the key attributes of one table to the related table Consider relationship type Video IsCopyOf Movie –Key of Movie is movieId –Add attribute movieId to table Video Schema Video (videoId int, [other attributes omitted] movieId int references Movie) Attribute movieId of relation Video is called a foreign key –Because it’s value is the value of the key of an entity in another (foreign) table. –Each video is related to the movie with the same movieId

10. Copyright © 2003 Addison-Wesley One-to-Many RelationshipTypes For a one-to-many relationship type –Add the key attributes of one entity class to the other entity class (foreign key attributes). –Add the foreign key attributes to the class whose cardinality is 1 Rule 3: For each one-to-many relationship type R between subject class S and target class T –add the key attributes of class S to class T as foreign keys –Name the attributes using the role that S plays in relationship type R –Add the attributes of the relationship type R to target class T.

11. Copyright © 2003 Addison-Wesley One-to-One Relationship Types The foreign key attributes may be added to either schema –Each entity class is to-one in the relationship type Choose which class to include the foreign key attributes –One option is to try to minimize the number of null values Rule 4: For each one-to-one relationship type between two classes, choose one class to be the subject and one to be the target –Add the key attributes of the subject class to the target schema as foreign key attributes –Add the attributes of the relationship type to the target schema, just as in Rule 3

12. Copyright © 2003 Addison-Wesley Many-to-Many Relationship Types ssnstoreId 358-44-78653 579-98-87785 358-44-78655 Many-to-Many relationship types between 2 classes cannot be represented as simple attributes in either related table Rule 5: For each many-to-many relationship type R between classes S and T –Create a new relation schema R –Add attributes to represent the key of S and the key of T as foreign key attributes –The key of schema R is the combination of those attributes –Add the relationship attributes to schema R, as in Rule 3 Schema: WorksIn (ssn string references Employee, storeId number references Store)

13. Copyright © 2003 Addison-Wesley Weak Entity Classes Weak Entity classes have no keys of their own Create keys from –Foreign keys of identifying relationship types –Partial keys of the weak class Rule 6: For each weak entity class W –Create a new relation schema with the same name –For each identifying relationship, Add the key attributes of the related class to the new schema as foreign key attributes –Declare the key of the schema to be the combination of the foreign key attributes and The partial key attributes of the weak entity class –Add the simple and composite attributes of class W to the schema, as in Rules 2 and 3 Relation Rental, shown at right, still must add foreign key for Customer

14. Copyright © 2003 Addison-Wesley Multi-valued Attributes Represent each multi-valued attribute as if it were a weak entity class with –identifying relationship with owner class –All composite attributes of multi-valued attribute are partial keys Rule 7: For each multi-valued attribute M of an entity class C –Define a new relation schema M –Add the components of attribute M to the new schema –Add the key attributes of the schema that contains the other attributes of C to M as a foreign key –Define the key of the new schema to be the combination of all of its attributes. The diagram below shows attribute otherUsers represented as a weak entity class and as a schema –Do not modify the E-R diagram. This diagram is included to illustrate the concept.

15. Copyright © 2003 Addison-Wesley Inheritance First representation strategy –Represent each superclass and each subclass as individual tables Each table has the attributes of the corresponding class Each subclass table has the key of the superclass as both key and foreign key Rule 8a: –Create a relation schema for each superclass C using rules 1 and 2. For each subclass of C that has a defining attribute, add that attribute to the schema for C. –For each subclass S, create a new relation schema. Add the simple and composite attributes of class S to the schema, as in Rules 1b and 2. Add the key of the superclass C as a foreign key referencing relation C. Declare the key of the subclass relation for S to be this foreign keys

16. Copyright © 2003 Addison-Wesley Example of Rule 8a

17. Copyright © 2003 Addison-Wesley Case in Point Relational Model for Video Sales for BigHit Online Process –Evaluate E-R model for BigHit Online Figure 4.25, repeated as Fig. 5.23 –Transform E-R diagram into relation schemas Apply rules 1-8 –Evaluate relation schemas for clarity, accuracy and completeness

18. Copyright © 2003 Addison-Wesley E-R Diagram for BigHit Online

19. Copyright © 2003 Addison-Wesley Creating Relation Schemas After applying Rule 1, before adding relationship types and inheritance –Customer: (accountId number, lastName string, firstName string) –Sale: (saleId number, dateSold date, totalCost number) –Movie: (movieId number, title string, genre string) –ShoppingCart: (cartId number, dateCreated date) Adding composite attributes to Sale –Sale: (saleId number, dateSold date, totalCost number, creditCardType string, creditCardExpiration date, creditCardAccountNumber number) Adding relationship type Purchases to Sale –Sale: (saleId number, dateSold date, totalCost number, creditCardType string, creditCardExpiration date, creditCardAccountNumber number, accountId number references Customer)

20. Copyright © 2003 Addison-Wesley Adding Relationship Types Add Selects to ShoppingCart –ShoppingCart: (cartId number, dateCreated date, accountId number references Customer) Add schemas for many-to-many relationship types –SaleItem: (saleId number references Sale, movieId references Movie, quantity number) –CartItem: (cartId number references ShoppingCart, movieId references Movie, quantity number) Add schemas for multi-valued attributes –CreditCards: (accountId number references Customer, accountNumber number, type string, expiration date) –ShippingAddresses: (accountId number references Customer, street string, city string, state string, zipcode string) Add schemas for subclasses of Movie –DVD: (movieId number references Movie, videoFormat string, languages string, captioning string, cost currency, quantity number) –Videotape: (movieId number references Movie, format string, soundtrack string, cost currency, quantity number)

21. Copyright © 2003 Addison-Wesley Resulting Database Schema for BigHit Online Customer: (accountId number, lastName string, firstName string) Sale: (saleId number, dateSold date, totalCost number, creditCardType string, creditCardExpiration date, creditCardAccountNumber number, accountId number references Customer) Movie: (movieId number, title string, genre string) ShoppingCart: (cartId number, dateCreated date, accountId number references Customer) SaleItem: (saleId number references Sale, movieId references Movie, quantity number) CartItem: (cartId number references ShoppingCart, movieId references Movie, quantity number) CreditCards: (accountId number references Customer, accountNumber number, type string, expiration date) ShippingAddresses: (accountId number references Customer, street string, city string, state string, zipcode string) DVD: (movieId number references Movie, videoFormat string, languages string, captioning string) Videotape: (movieId number references Movie, format string, soundtrack string)