A four-way Relationship• The relation Contracts involvestwo studios, a star, and a movie.

• The intent is that one studio,having a certain star undercontract, may further contract witha second studio to allow that starto act in a particular movie.

• We can visualize the relationshipwith the following 4-tuple:

(studio1, studio2, star, movie)

Stars Movies

Contracts

Studios

Studio-of-stars

Producing-studio

• Given a star, a movie, and a studio producing the movie, there can be onlyone studio that “owns” the star.

• Similarly, given a star, a movie, and the star’s studio, we can determine aunique producing studio.

• Given a star, the star’s studio, and a producing studio, there could beseveral different contracts allowing the star to act in several movies.

• Similarly, a producing studio might contract with some other studio to usemore than one of their stars in one movie.

• Not all of the other entities are needed to uniquely identify the remainingentity.

• E.g. only movie+star is needed to find the prod. stud. or the ownerstudio.

About the Arrows

• For a variety of reasons, like translating to the UML, we might want to haveonly binary relationships.

• Notably, any multiway relationship can be converted to a set of binarymany-one relationships.• When removing a multiway relationship, we introduce a corresponding new

connecting entity set, which has many-one relationships with the participatingentity sets in the original relationship.

• The tuples (triples, quadruplets, etc) of the original relationship become entitiesof the new entity set.

Converting Multiway Rel. to Binary

Stars Movies Star-of Movies-of

Contracts

Studio-ofstar

Producing_studio

Studios

• Often, a class contains certain objects that have special properties notassociated with all members of the class.

• If so, we find it useful to organize the class into subclasses, each subclasshaving its own special attributes and/or relationships.

• We relate the parent with child entity sets by a special (1-1) relationshipcalled isa.

Subclasses

Voices

to Stars

Movies

length title year filmType

Cartoons

isa isa

Murder-Mysteries

weapon

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 towhich they belong. The entity has whatever attributes any of its components has, and it

participates in whatever relationships its components participate in.

Inheritance in the E/R Model

Voices

to Stars

Movies

length title year filmType

Cartoons

isa isa

Murder-Mysteries

weapon

• Take the movie Roger Rabit, which is both a cartoon and murder-mystery.

• It will have components in all three entity sets: Movies, Cartoons, andMurder-Mysteries.

• The three components are connected together into one entity by the isarelationships.

• Roger Rabit will have all four attributes of Movies, the attributeweapon, and finally will participate in the relationship voices.

Components Example

Voices

to Stars

Movies

length title year filmType

Cartoons

isa isa

Murder-Mysteries

weapon

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 givenwhat we know about the part of the real world being modeled.

• If we define a relationship Stars-in between Stars and Movies, itshould be a many-many relationship.

• Not always obvious.– E.g. Courses and Instructors: What’s the multiplicity of a

relationship “Teaches”?

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”?

Stars MoviesContracts

Studios

Movies Stars-In Stars

Owns

Studios

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 nostars under contract for that movie, or no such contract is known to ourdatabase, then we must retain Owns.

Redundancy or Right Relationships II• Should we have a relationship “Works-for” between “Stars” and

“Studios”?

It depends.We need it if a starmight work for a

studio in a mannerunrelated to a movie.

Multiway relationships vs. Connecting Entity Sets• Let us suppose that contracts involve one star, one movie, but

any set of studios.

Stars MoviesContracts

Studios

Better, because we canrepresent the fact that acontract can involve notmore than one star andone movie, but many

movies.

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.

Keys, Example• Let’s consider the entity set Movie.

• We might assume that the attribute title is a key. However, therecan 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 ofMovie class, then the DBMS will not allow us to insertinformation 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.

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. Movietitle+year is also key for Cartoons.

Example: name is Key for Beers

Beers

Ales

isa

name manf

color

Example: a Multi-attribute Key

Courses

dept number hours room

• Note that hours and room could also serve as a key, but we must select only one key.

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 akey 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 iswidespread– Students ID’s in universities– Driver license numbers– Automobile registration numbers

Entity Sets Versus Attributes I• Example: Bad design

This design repeats the manufacturer’s address oncefor each beer;Loses the address if there are temporarily no beers fora manufacturer.

Beers

name manf manfAddr

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 anentity set because ofthe nonkey attributeaddr.•Beers deserves to be anentity set because it isthe “many” of the many-one relationshipManfBy.

Beers ManfsManfBy

name name addr

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 includestheir name, address, phone, and Social Security number. Accounts havenumbers, types (e.g., savings, checking) and balances. We also need to recordthe 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 ofphones. Remember that we do not allow attributes to have nonatomictypes, such as sets, in the E/R model.

– ! d) Further modify your diagram so that customers can have a set ofaddresses, and at each address there is a set of phones.

Exercises II• Exercise 2.1.3: Give an E/R diagram for a database recording information

about teams, players, and their fans, including:1. For each team, its name, its players, its team captain (one of its players),

and the colors of its uniform.2. For each player, his/her name.3. 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 arelationship “Led-by” among two players and a team. The intention is thatthis relationship set consists of triples (player1, player2, team) such thatplayer 1 played on the team at a time when some other player 2 was the teamcaptain.a) Draw the modification to the E/R diagram.

• Exercise 2.1.5 : Modify Exercise 2.1.3 to record for each player the historyof teams on which they have played, including the start date and ending date(if they were traded) for each such team.

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 includestheir name (an attribute) and the following relationships: mother, father, andchildren. Give an E/R diagram involving the Person entity set and all therelationships in which it is involved. Include relationships for mother, father,and children. Do not forget to indicate roles when an entity set is used morethan once in a relationship.

• ! Exercise 2.1.7 : Modify your “people" database design of Exercise 2.1.6 toinclude the following special types of people:1. Females.2. Males.3. People who are parents.

• You may wish to distinguish certain other kinds of people as well, sorelationships connect appropriate subclasses of people.