Entity-Relationship Model
Design Principles, Keys, Subclasses, Exercises
Design Principles: Faithfulness
• 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”?
– In such cases, please state your assumptions.
Redundancy or Right Relationships
• 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.
Redundancy or Right Relationships
• Say everything once only! …well, not always obvious.
• Do we still need the two-ways relationships “Owns” and “Stars-In”?
Movies
Stars-In
Stars
Owns
Studios
Stars
Contracts
Studios
Movies
Redundancy or Right Relationships
• 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.
Multiway relationships vs. Connecting Entity Sets
• Let’s suppose that contracts involve one star, one movie,
but any set of studios.
With this we can
represent the fact that a
contract can involve not
more than one star and
one movie, but many
studios.
Stars
Contracts
Studios
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’s allowed for two entities to agree on some, but not all, of
the key attributes.
– We must designate a key for every entity set.
• In E/R, we underline the key attribute(s).
name
Beers
manf
Key for Movies
• 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.
Keys for Studios and Stars
• For Studios:
– Reasonable to assume that there are no two studios having the
same name.
– So, we will enforce name to serve as a key.
• For Stars:
– We may think that the name can’t serve to distinguish two people,
but…
– Yes! For stars the name distinguishes them since traditionally they
choose “stage names”.
– So, again here, we will enforce name to serve as a key.
Surrogate Keys
• Often, people introduce attributes whose role is to serve as a
key for classes.
– Companies assign employee ID’s to all employees, and these ID’s
are carefully chosen to be unique numbers.
– In Canada everyone has a SIN.
– Students ID’s in universities
– Driver license numbers
– Automobile registration numbers
Entity Sets Versus Attributes I
• Example: Bad design
name
manf
manfAddr
Beers
1. This design repeats the manufacturer’s
address once for each beer;
2. Loses the address if there are temporarily no
beers for a manufacturer.
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
name
Beers
name
ManfBy
Manfs
addr
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.
Subclasses
•
Often, a class contains certain objects that have special properties not
associated with all members of the class.
•
If so, organize the class into subclasses, each subclass having its
own special attributes and/or relationships.
length
to Stars
title
year
Relate parent with
child by a special
(1-1) relationship
called isa.
filmType
Movies
Voices
isa
Cartoons
isa
weapon
MurderMysteries
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.
length
to Stars
title
year
filmType
Movies
Voices
isa
Cartoons
isa
weapon
MurderMysteries
Components Example
•
Take the movie Roger Rabit, which is both a cartoon and murdermystery.
•
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.
length
to Stars
title
year
filmType
Movies
Voices
isa
Cartoons
isa
weapon
MurderMysteries
Keys for entity set hierarchies
In entity set hierarchies the key at root is key for all.
{title,year} is the key for Movies, Cartoons and Murder-Mysteries.
length
to Stars
title
year
filmType
Movies
Voices
isa
Cartoons
isa
weapon
MurderMysteries
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.
Exercises II
•
Exercise 2.1.3: Give an E/R diagram for a database recording
information about teams, players, and their fans, including:
1.
2.
3.
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.
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:
1. Females.
2. Males.
3. People who are parents.
•
You may wish to distinguish certain other kinds of people as well, so
relationships connect appropriate subclasses of people.