A four-way Relationship

1. Stars Movies Contracts Studio-of- stars Producing- studio Studios A four-way Relationship • The relationContractsinvolves two studios, a star, and a movie. • The intent is that one studio, having a certain star under contract, may further contract with a second studio to allow that star to act in a particular movie. • We can visualize the relationship with the following 4-tuple: • (studio1, studio2, star, movie)

2. About the Arrows • Given a star, a movie, and a studio producing the movie, there can be only one studio that “owns” the star. • Similarly, given a star, a movie, and the star’s studio, we can determine a unique producing studio. • Given a star, the star’s studio, and a producing studio, there could be several different contracts allowing the star to act in several movies. • Similarly, a producing studio might contract with some other studio to use more than one of their stars in one movie. • Not all of the other entities are needed to uniquely identify the remaining entity. • E.g. only movie+star is needed to find the prod. stud. or the owner studio.

3. Star-of Movies-of Stars Movies Contracts Studio-of star Producing_ studio Studios Converting Multiway Rel. to Binary • For a variety of reasons, like translating to the UML, we might want to have only binary relationships. • Notably, any multiway relationship can be converted to a set of binary many-one relationships. • When removing a multiway relationship, we introduce a corresponding new connecting entity set, which has many-one relationships with the participating entity sets in the original relationship. • The tuples (triples, quadruplets, etc) of the original relationship become entities of the new entity set.

4. length title year filmType to Stars Movies Voices isa isa weapon Cartoons Murder- Mysteries Subclasses • Often, a class contains certain objects that have special properties not associated with all members of the class. • If so, we find it useful to organize the class into subclasses, each subclass having its own special attributes and/or relationships. • We relate the parent with child entity sets by a special (1-1) relationship called isa.

5. length title year filmType to Stars Movies Voices isa isa weapon Cartoons Murder- Mysteries Inheritance in the E/R Model • In the object-oriented world, objects are in one class only. • Subclasses inherit properties from superclasses. • In contrast, E/R entities have components in all subclasses to which they belong. • The entity has whatever attributes any of its components has, and it participates in whatever relationships its components participate in.

6. length title year filmType to Stars Movies Voices isa isa weapon Cartoons Murder- Mysteries Components Example • Take the movie Roger Rabit, which is both a cartoon and murder-mystery. • It will have components in all three entity sets: Movies, Cartoons, and Murder-Mysteries. • The three components are connected together into one entity by the isa relationships. • Roger Rabit will have all four attributes of Movies, the attribute weapon, and finally will participate in the relationship voices.

7. Design Principles: Faithfulness • That is, entity sets and their attributes should reflect reality. • Can't attach an attribute “number-of-cylinders” to Stars • Whatever relationships are asserted should make sense given what we know about the part of the real world being modeled. • If we define a relationship Stars-in between Stars and Movies, it should be a many-many relationship. • Not always obvious. • E.g. Courses and Instructors: What’s the multiplicity of a relationship “Teaches”?

8. Movies Stars-In Stars Contracts Owns Movies Stars Studios Studios Redundancy or Right Relationships • Say everything once only! • …well, not always obvious. • Suppose we decided that we needed the three-way relationship “Contracts.” Do we still need the two-ways relationships “Owns” and “Stars-In”?

9. Redundancy or Right Relationships II • Can it be possible to deduce the relationship “Owns” from “Contracts”? • If for every movie, there is at least one contract involving: • that movie, • its owning studio, and • some star for that movie, then we can dispense with Owns. • However, if there is the possibility that a studio owns a movie, yet has no stars under contract for that movie, or no such contract is known to our database, then we must retain Owns.

10. Redundancy or Right Relationships II • Should we have a relationship “Works-for” between “Stars” and “Studios”? It depends. We need it if a star might work for a studio in a manner unrelated to a movie.

11. Contracts Movies Stars Studios Multiway relationships vs. Connecting Entity Sets • Let us suppose that contracts involve one star, one movie, but any set of studios. Better, because we can represent the fact that a contract can involve not more than one star and one movie, but many movies.

12. Keys • A key is a set of attributes for one entity set such that no two entities in this set agree on all the attributes of the key. • It is allowed for two entities to agree on some, but not all, of the key attributes. • We must designate a key for every entity set. • Underline the key attribute(s). • In an ISA hierarchy, only the root entity set has a key, and it must serve as the key for all entities in the hierarchy.

13. Keys, Example • Let’s consider the entity set Movie. • We might assume that the attribute title is a key. However, there can be different movies with the same name: • “Godzilla” has several different versions (Japanese, American etc.). • If we enforce in the database a key constraint on attribute title of Movie class, then the DBMS will not allow us to insert information about different “Godzilla’s”. • A better choice is to take the set {title, year} of attributes as a key. • We still run the risk that there are two movies made in the same year, with the same title, but that’s very unlikely.

14. Keys, Example (Continued) • For the entity set Studios: • It’s reasonable to assume that there are no studios with the same name. • So, we will enforce name to serve as a key. • For the entity set Stars: • We may think that the name cannot serve to distinguish two people, but… • Yes! For stars the name distinguish them since they traditionally choose “stage names”. • So, again here, we will enforce name to serve as a key. • Note. In entity set hierarchies the key at root is key for all. Movie title+year is also key for Cartoons.

15. Beers name manf isa Ales color Example: name is Key for Beers

16. dept number hours room Courses Example: a Multi-attribute Key • Note that hours and room could also serve as a • key, but we must select only one key.

17. Surrogate Keys • We might think that it is difficult to find keys or to be sure that a set of attributes forms a key. In practice the matter is usually simpler. • In the real world, people introduce attributes whose role is to serve as a key for classes. • For example companies generally assign employee ID’s to all employees, and this ID’s are carefully chosen to be unique numbers. • In Canada everyone has a SIN. • There is nothing wrong with there being several choices of key for a class. • The idea of creating an attribute whose purpose is to serve as a key is widespread • Students ID’s in universities • Driver license numbers • Automobile registration numbers

18. name manf manfAddr Beers Entity Sets Versus Attributes I • Example: Bad design This design repeats the manufacturer’s address once for each beer; Loses the address if there are temporarily no beers for a manufacturer.

19. name name addr ManfBy Beers Manfs Entity Sets Versus Attributes II • An entity set should satisfy at least one of the following conditions: • It is more than the name of something; it has at least one nonkey attribute. or • It is the “many” in a many-one or many-many relationship. • Example: Good • Manfs deserves to be an entity set because of the nonkey attribute addr. • Beers deserves to be an entity set because it is the “many” of the many-one relationship ManfBy.

20. Exercises I • Exercise 2.1.1: Let us design a database for a bank, including information about customers and their accounts. Information about a customer includes their name, address, phone, and Social Security number. Accounts have numbers, types (e.g., savings, checking) and balances. We also need to record the customer(s) who own an account. Draw the E/R diagram for this database. • Exercise 2.1.2: Modify your solution to Exercise 2.1.1 as follows: • a) Change your diagram so an account can have only one customer. • b) Further change your diagram so a customer can have only one account. • ! c) Change your original diagram of Exercise 2.1.1 so that a customer can have a set of addresses (which are street-city-state triples) and a set of phones. Remember that we do not allow attributes to have nonatomic types, such as sets, in the E/R model. • ! d) Further modify your diagram so that customers can have a set of addresses, and at each address there is a set of phones.

21. Exercises II • Exercise 2.1.3: Give an E/R diagram for a database recording information about teams, players, and their fans, including: • For each team, its name, its players, its team captain (one of its players), and the colors of its uniform. • For each player, his/her name. • For each fan, his/her name, favorite teams, favorite players, and favorite color. • Exercise 2.1.4: Suppose we wish to add to the schema of Exercise 2.1.3 a relationship “Led-by” among two players and a team. The intention is that this relationship set consists of triples (player1, player2, team) such that player 1 played on the team at a time when some other player 2 was the team captain. a) Draw the modification to the E/R diagram. • Exercise 2.1.5 : Modify Exercise 2.1.3 to record for each player the history of teams on which they have played, including the start date and ending date (if they were traded) for each such team.

22. Exercises III • ! Exercise 2.1.6 : Suppose we wish to keep a genealogy. We shall have one entity set, Person. The information we wish to record about persons includes their name (an attribute) and the following relationships: mother, father, and children. Give an E/R diagram involving the Person entity set and all the relationships in which it is involved. Include relationships for mother, father, and children. Do not forget to indicate roles when an entity set is used more than once in a relationship. • ! Exercise 2.1.7 : Modify your “people" database design of Exercise 2.1.6 to include the following special types of people: • Females. • Males. • People who are parents. • You may wish to distinguish certain other kinds of people as well, so relationships connect appropriate subclasses of people.