1. Entities and Relationships
Database Principles

2. In sentences, relationships often show up as verbs.
What is a Relationship?
An association between or among entities. Can be binary, ternary, quartic, etc.
Because entities are “types” of things, relationships also have to be “types” of associations.
Entities have instances and relationships have occurrences. Entities are while relationships happen.
In sentences, relationships often show up as verbs.
Is a relationship between the entities PERSON and AUTOMOBILE.
is an occurrence of this relationship.
Most People own Automobiles
Andrew Pletch owns a VW Passat with VIN:
Database Principles

3. How do we picture a Relationship?
PERSON
AUTOMOBILE
OWNS
As a line joining the related entities with a diamond in the middle
The name of the relationship goes in or on top of the diamond
The relationship name tends to make grammatical sense in one direction only
Database Principles

4. Just to make it clear: Database Principles This is a relationship
PERSON
AUTOMOBILE
OWNS
This is a collection of occurrences of this relationship
Database Principles

5. Can Relationships have Properties?
Yes!
They are also called attributes.
They appear in the diagram underneath the diamond.
Why can’t PurchaseDate be an attribute of AUTOMOBILE?
Reason 1: Because the same car can be sold several times and so have a list of PurchaseDates
Reason 2: Because PurchaseDate only makes sense in the context of “what car?” and “who bought it?”
PERSON
AUTOMOBILE
OWNS
PurchaseDate ?
Database Principles

6. Attributes Don’t Get Copied.
The fact that a vehicle has an owner who has a name is modeled by the <owns> relationship.
We do NOT replicate this fact by also putting an extra attribute in the VEHICLE entity.
PERSON
AUTOMOBILE
OWNS
SSN
FName
LName
DOB
VIN
OwnerName
Make
Model
Year
NOT permitted/recommended
Database Principles

7. Does a Relationship have a Key?
Yes, but they are not documented in the relationship.
Typically it is the combined keys of the participating entities.
The key to <owns> is the pair (PersonID,VIN).
We do NOT duplicate these atrributes anywhere else in the diagram.
No two occurrences of <owns> have the same key values.
AUTOMOBILE
OWNS
PurchaseDate
PERSON
VIN
PersonID
Database Principles

8. What does <owns> mean?
Exercise:
What does <owns> mean?
What are some occurrences of <owns>?
Describe a general occurrence of <owns>.
Based on the model, can the same person <own> the same vehicle more than once? Why not?
What would it mean to <own> the same vehicle more than once? Give a couple of examples.
How would we change the model if this kind of multiple ownership needed to be modeled?
Database Principles

9. What if you CAN <own> the same car more than once?
That’s possible!
But you need the purchase date to tell these two events apart
Key to <owns> now: (PersonID,VIN,PurchaseDate).
No two occurrences of <owns> have the same key values.
We underscore any attribute of a relationship that is part of the relationship key.
OWNS
PERSON
AUTOMOBILE
PersonID
VIN
PurchaseDate
Database Principles

10. How do the two versions of <owns> differ?
Exercise:
How do the two versions of <owns> differ?
What are some occurrences of the new <owns>?
Describe a general occurrence of the new <owns>.
Database Principles

11. Relationship Cardinalities:
Read the notes:
Enterprise Rules are used by the database designer determine what things are entities, what things are relationships and what things are attributes.
Example: Every Employee earns one and only one salary
EMPLOYEE ID
. . .
Salary
single-valued fact about all employees
so is modeled as an attribute of EMPLOYEE
Database Principles

12. Relationship Enterprise Rules:
AUTOMOBILE
OWNS
purchase_date
PERSON
VIN
PersonID
How do we model the following rules?
The answer seems to boil down to asking how many times each PERSON or each AUTOMOBILE can participate in the <owns> relationship.
Rule 1: A person can own one and only one AUTOMOBILE
Rule 2: A person can own many automobiles acquired at any time
Rule 3: Some people do not own automobiles at all
Rule 4: Every automobile has at least one and possibly many owners.
Database Principles

13. How many vehicles can a person own?
Rule 1 and Rule 2 on the previous slide are mutually exclusive but our picture doesn’t allow us to say which of these is the rule that applies in this case.
That is because the picture is not complete.
We complete the picture by adding what we call Participation Numbers to each entities participation in each relationship.
Database Principles

14. Minimum Participation Numbers:
AUTOMOBILE
OWNS
purchase_date
PERSON
VIN
PersonID
Does every PERSON own at least one AUTOMOBILE?
Is every AUTOMOBILE owned by at least one PERSON?
(0,?)
(1,?) √
no – min participation number (mPN) is 0
yes – min participation number (mPN) is 1
no – min participation number (mPN) is 0
yes – min participation number (mPN) is 1 √
Database Principles

15. Maximum Participation Numbers:
AUTOMOBILE
OWNS
purchase_date
PERSON
VIN
PersonID
Does any PERSON own more than one AUTOMOBILE?
Is any AUTOMOBILE owned by more than one PERSON?
(0,n)
(0,n)
(1,1)
(1,1)
no – max participation number (MPN) is 1
yes – max participation number (MPN) is n √
no – max participation number (MPN) is 1
yes – max participation number (MPN) is n √
Database Principles

16. Interpreting Participation Numbers:
AUTOMOBILE
OWNS
purchase_date
PERSON
VIN
PersonID
(1,1)
(0,n)
mPN = 1:
MPN = 1:
mPN = 0:
MPN = n:
Every PERSON <owns> at least one AUTOMOBILE
No PERSON <owns> more than one AUTOMOBILE
Not every AUTOMOBILE <is owned by> a PERSON
Some AUTOMOBILES <may be owned by> more than one PERSON
Database Principles

17. Participation Numbers and Keys
Compare
and
AUTOMOBILE
OWNS
PurchaseDate
PERSON
VIN
PersonID
(0,n)
(1,n)
AUTOMOBILE
OWNS
PurchaseDate
PERSON
VIN
PersonID
(0,n)
(1,n)
The first lets a PERSON own multiple AUTOMOBILES but not
the same AUTOMOBILE twice.
The latter allows both things to happen.
Database Principles

18. An Example: Database Principles
A library keeps records of current loans of books to borrowers.
Each borrower has a borrower# and each copy of a book has
an accession# (there may be several copies of the same book).
The library keeps the name and address of each borrower so
that overdue reminders can be sent if necessary.
For each book, the library keeps the title, authors, publisher,
publication date, ISBN, purchase price and current list price.
Borrowers can have one of two statuses - junior and senior.
There are restrictions on the number of books a borrower may
take out at one time depending on his/her status.
Books which are out on loan may be reserved by other borrowers.
The library does not buy paperbacks. When a new edition of a book
is acquired, all copies of earlier editions are removed from the shelves.
Database Principles

19. List all Nouns: Database Principles Library Loan Book
Borrower
Borrowerid
Copy
Accession_no
B_name
B_address
Reminder
Title
Author
Publisher
Pub_date
ISBN
P_price
C_price
B_status
Loan_limit
Reservation
A library keeps records of current loans of books to borrowers.
Each borrower has a borrower# and each copy of a book has
an accession# (there may be several copies of the same book).
The library keeps the name and address of each borrower so
that overdue reminders can be sent if necessary.
For each book, the library keeps the title, authors, publisher,
publication date, ISBN, purchase price and current list price.
Borrowers can have one of two statuses - junior and senior.
There is a loan limit on the number of books a borrower may
take out at one time depending on his/her status.
Books which are out on loan may be reserved by other borrowers.
When a book is returned the person who made the earliest
Reservation for he book is notified.
Database Principles

20. Which Nouns are Entities and Which are Attributes?
Library
Loan
Book
Borrower
Borrowerid
Copy
Accession_no
B_name
B_address
Reminder
Title
Author
Publisher
Pub_date
ISBN
P_price
C_price
B_status
Loan_limit
Reservation
Why is Author not an entity?
(i) It certainly is a property of Book.
(ii) All we know about authors are their name.
(iii) Authors do nothing but author books.
(iv) In a Publishing House database Authors
would be entities.
(v) In a Library database, Authors only exist as
authors of Books so do not have “independent”
existence.
Database Principles

21. List Entities and their Attributes:
Loan
l_date how else do we know if it is overdue
Book
ISBN
Author
Title
Pub_date
C_price
Borrower
Borrowerid
B_name
B_address
B_status
Loan_limit
Publisher
Pub_name
Reminder
Copy
Accession_no
P_price
Reservation
R_date how else do we send a notice to the earliest reservation
Why is Library dropped altogether?
(i) Although certainly an entity, there is only one instance.
(ii) All other entities have multiple instances.
(iii) No properties exist for Library other than its name.
(iv) If our application was a library system with many
libraries then it would make sense to make Library
an entity.
Database Principles

22. First Look at a List of Entities:
NOTES:
1: Things that have keys
that belong to them are
likely to be entities.
2: Nouns that are missing
keys and seem to relate to
other things are likely to be
relationships and not entities.
Database Principles

23. Now Add Some Relationships
1: Reminder is more of
a transaction or activity
than a thing. All the info
needed for a Reminder
is found in other things.
2: Pub_name can be an
attribute of Book if we drop
the Publisher entity
Database Principles

24. Final Version (without mMpn):
Database Principles

25. Final Version (with mMpn)
Does every Cardholder reserve a book?
Does any Cardholder reserve more than
one book?
Is every Book reserved by at least one
Cardholder?
Is any Book reserved by more than one
Does every Cardholder borrow a book?
Does any Cardholder borrow more than
Is every Copy borrowed by at least one
Is any Cardholder borrowed by more than one
Cardholder? [current loans only]
Does every Book exist as a copy in the
Library?
Does any Book have more than one copy
in the Library?
Is every Copy a copy of some Book?
Is any Copy a copy of more than one
Book?
(n=0)
y=n
(n=0)
(y=n)
(n=0)
(y=n)
(n=0)
(n=1)
(y=1)
(y=n)
(y=1)
(n=1)
1: Cardholder not
Borrower (more accurate)
Database Principles

26. A Practical Approach to ER Design
Read the “Ten Steps to Database Design” web page. It shows how to complete the task of preparing an External View ER diagram in a systematic fashion.
Database Principles

27. More on Participation Numbers:
Some relationships are ternary: Consider vaccines produced by different manufacturers and sold in different countries.
The problem is, how to ask the PN questions.
VACCINE
V_ID
MANUFACTURER
Man_ID
COUNTRY
Name
distributed by, in
Database Principles

28. Asking the Right Question:
VACCINE
V_ID
MANUFACTURER
Man_ID
COUNTRY
Name
distributed by, in
(1,n)
(c,d)
(a,b)
(1,n)
a? : Does every VACCINE participate at least once in
the <distributed by, in> relationship? (n=0, y=1)
b? : Does any VACCINE participate more than once in
the <distributed by, in> relationship? (n=1, y=n)
c? :
d? :
e? :
f? :
(e,f)
(1,n)
Does every MANUFACTURER <distribute> at least
one VACCINE in some COUNTRY?
Does any MANUFACTURER <distribute> more than
one VACCINE in some COUNTRY?
Is every COUNTRY the recipient of at least
one VACCINE manufactured by someone?
Is any COUNTRY the recipient of more than
one VACCINE manufactured by someone?
Database Principles

29. A Class Example: Part A:
An Airline has planes that are used to fly scheduled flights.
Flight segments are between two airports – departure and arrival. They
are scheduled to depart at a certain time and arrive at a certain time. The
type of plane to be used is also known.
Scheduled flights are a sequence of flight segments.
Actual flights occur on particular dates using a particular plane flown
by a particular pilot and co-pilot. They take off at a given time (perhaps)
different from the scheduled departure time and have an ETA.
Pilots have names, SSNs, addresses, DOB.
Planes have unique numbers, capacities – both1st class and economy
This database doesn’t deal with passengers.
Database Principles

30. A Class Example: Part B:
An Airline reservation system keeps information about customers,
their reservations, tickets, seat assignments for scheduled flights.
Scheduled flights are between two airports – departure and arrival. They
are scheduled to depart at a certain time and arrive at a certain time. The
type of plane to be used is also known. Scheduled flights have flight
numbers.
Customer info includes name, address, address, phone number
A reservation is for a certain customer on a sequence of scheduled flights. It
has a locator number. Each reservation has a single source and single
destination airport.
A ticket is a paid reservation. It has a ticket number and a payment method
Including credit card info
Database Principles

31. An entity is weak if it depends on another entity for part of its key.
Weak Entities
An entity is weak if it depends on another entity for part of its key.
Remember, we can’t copy keys around an ER diagram.
We can’t do the following
But because the key to an Order Line is really the combination of (OrderNumber,LineNumber), Line instances depend on the key to Order for part of their key.
Database Principles

32. Weak Entities (cont):
Instead we draw a double line around the weaker (dependent) entity and the relationship that it depends on.
This picture says that Line is a weak entity whose key consists of the pair (OrderNumber, LineNumber).
Weak entities always have a (1,1) participation number pair linking to the entity they depend upon. Why?
Database Principles

33. Weak Entity – Another Example
Some times a relationship from one department’s point a view is an entity in some other department.
For the Registrar’s Office, Enrollment as a relationship between two entities – Student and Course:
For the Bursar’s Office, Enrollment is an entity that relates to a Chargeable Item.
Why are <enrolls> and Enrollment are the same thing???
Database Principles

34. How to merge the two diagrams?
become
NOTE: We converted a
relationship into a weak-
entity relationship
Registrar’s View
Bursar’s View
Database Principles

35. A New Model for an Old Idea
The previous example used to be called aggregation. It used to be modeled as:
The box labeled Enrollment turned the <enrolls> relationship into an entity with the same key.
We don’t use this method any more because most modeling tools don’t support it.
Database Principles

36. Before inheritance became popular because of OOP there was IS_A.
Both Professor and Student possess the attributes ID, Name and DOB by “inheritance”.
The key to Professor and the key to Student is the key to Person.
Database Principles

37. This is clear for two reasons:
Modeling History:
The <borrows> relationship in the Library model only models current loans and not past loans.
This is clear for two reasons:
The same Copy can not be loaned more than once
Reason?
The same Cardholder can not borrow the same Copy more than once.
Database Principles

38. Modeling History 2:
Suppose we wanted to model a complete loan history of all loans; past and present.
To allow more than one loan of the same copy we would first need to change the copy Max PN to n.
To allow the same Cardholder to borrow the same Copy more than once (but on different occasions) we need to add time to the key. To show that this is part of the key to <borrows> and not just another attribute we underline it.
a copy can be loaned
out more than once
added to the key
Database Principles

39. Self-Related Entities:
Some entities are related to themselves.
This models what kind of a data structure?
Database Principles

40. More on Participation Numbers:
Participation Numbers are concise representations of Enterprise Rules.
A database designer, viewing mMPNs, can make positive declarations about the rules by which a business is run.
Participation numbers are flexible enough (16 different ways to model a binary relationship) to handle all we need to do.
Database Principles