1. The E-R Model CS 157A Lecture 4 Prof. Sin-Min Lee
Department of Computer Science

6. Entity-Relationship (E-R) Model
COURSE#
CTITILE
PROF
SCHED
ROOM
COURSE
ENROLL
GRADE
STUDENT
SNAME
CREDITS
STUID
MAJOR

7. Class Hierarchies
* Sometimes it’s natural to classify the entities in an entity set into subclass, then we will use Class Hierarchies.
name
lot
ssn
Employees
Hourly
wage
Hours
worked
ISA
contractid
Hourly
Emps
Contract
Emps

8. Aggregation * A relationship set is an association between
entity sets.
Sometimes we have to model a relationship between a collection of entities and relationships, then we need to use aggregation.
Aggregation
name
ssn
lot
Employees
monitors
until
Started-on
dname
since
pid
pbudget
did
budget
Sponsors
Projects
Department

9. Conceptual design using the ER model
Should a concept be modeled as an entity or an attribute ?
Should a concept be modeled as an entity or a relationship ?
What are the relationships ? Should we use binary or ternary relationships ?
Should we use aggregation ?

10. UNARY RELATIONSHIP
Arthur Keller – CS 180
Winter 2002
Unary relationship : is an associate of 1 entity set.
* Note : an entity set is a group of related entities.
Dr. Lee
Student
* Entity set =
* Relation =
* Attribute =

11. BINARY RELATIONSHIP
Binary relationship : is an associate of 2 entities sets .
Works_In
Employees
Department
ssn
name
lot
dname
budget
did
* Entity set =
* Relation =
* Attribute =

12. TERNARY RELATIONSHIP
Ternary relationship : is an associate of 3 entities sets .
Works_In
Employees
Department
ssn
name
lot
did
budget
dname
Locations
add
cap
* Entity set =
* Relation =
* Attribute =

13. QUARTERY RELATIONSHIP
Quartery relationship : is an associate of 4 entities sets.
Product
Works_In
Employees
Department
Locations
* Entity set =
* Relation =
* Attribute =

15. Additional features of the ER model
In the ER model allow us to draw important distinctions about the data.
One of the features of the ER model is :
Key Constraints
Note : A key constraints is also known as Primary key.
A primary key is a candidate key selected to uniquely identify all other attribute values in any given row, can not contain null entries.

16. An example of Key Constraints on Manage
name
Since
dname
budget
did
ssn
lot
Manages
Department
Employees
ssn = Social Security Number.
did = Department id.

18. Weak Entities name cost age ssn lot Policy Department Employees
Pname
age
ssn
lot
Policy
Department
Employees
A weak entity : is an entity of which is existence depends on other entities.

21. Entity versus Attribute
from to
name
dname
budget
did
ssn
lot
Works-In2
Department
Employees
* Entity set =
* Relation =
* Attribute =

22. Entity versus Relationship
dname
name
did
budget
ssn
lot
Manages
Department
Employees
since
apptnum
Mgr-Appts
dbudget

23. Binary versus Ternary Relationship
name
Pname
age
ssn
lot
Covers
Dependents
Employees
Policies
policyid
cost

24. Using a Ternary Relationship instead of Aggregation
Started-on
dname
did
budget
pid
pbudget
Sponsors
Departments
Projects
Employees
lot
ssn
name

25. ONE TO MANY RELATIONSHIP
customer-name
customer-street
loan-number
amount
customer-city
customer-id
borrower
loan
customer

26. MANY TO ONE RELATIONSHIP
customer-name
customer-street
loan-number
amount
customer-city
customer-id
borrower
loan
customer

27. ONE TO ONE RELATIONSHIP
customer-name
customer-street
loan-number
amount
customer-city
customer-id
borrower
loan
customer

28. E-R Diagram with Composite, Multivalued, and Derived Attributes
street-name
middle-name
Street-number
apartment-number
last-name
first-name
street
name
city
customer-id
address
state
Customer
zip-code
age
date-of-birth
phone-number

29. WEAK ENTITY SETS
An entity set may not have sufficient attributes to form a primary key. Such an entity set is termed a weak entity set. An entity set that has a primary key is termed a strong entity set.
For a weak entity set to be meaningful, it must be associated with another entity set, called the identifying or owner entity set. The relationship associating the weak entity set with the identifying entity set is called the identifying relationship.
The identifying relationship is many to one from the weak entity set to the identifying entity set, and the participation of the weak entity set in the relationship is total.

30. DISCRIMINATOR
The discriminator of a weak entity set is a set of attributes that allows this distinction to be made. For example, the discriminator of a weak entity set payment is the attribute payment-number, since, for each loan a payment number uniquely identifies one single payment for that loan. The discriminator of a weak entity set is also called the partial key of the entity set.
Note: although each payment entity is distinct, payments for different loans may share the same payment-number. Thus, payment entity set does not have a primary key; it is a weak entity set.
The primary key of a weak entity set is formed by the primary key of the identifying entity set, plus the weak entity set’s discriminator.

31. E-R DIAGRAM WITH A WEAK ENTITY SET
In a E-R diagrams, a doubly outlined box indicates a weak entity set, and a doubly outlined diamond indicates the corresponding identifying relationship. We underline the discriminator of a weak entity set with a dashed line.
payment-date
amount
loan-number
payment-number
payment-amount
loan-payment
loan
payment
E-R diagram with a weak entity set

36. Entity/Relationship Model
Diagrams to represent designs.
Entity like object, = “thing.”
Entity set like class = set of “similar” entities/objects.
Attribute = property of entities in an entity set, similar to fields of a struct.
In diagrams, entity set  rectangle; attribute  oval.
name
phone ID
Students
height

37. Relationships Connect two or more entity sets.
Represented by diamonds.
Taking
Students
Courses

38. Relationship Set
Think of the “value” of a relationship set as a table.
One column for each of the connected entity sets.
One row for each list of entities, one from each set, that are connected by the relationship.
Students Courses
Sally CS180
Sally CS111
Joe CS180
… …

39. Multiway Relationships
Usually binary relationships (connecting two E.S.) suffice.
However, there are some cases where three or more E.S. must be connected by one relationship.
Example: relationship among students, courses, TA's (and graders).
Possibly, this E/R diagram is OK:
Taking
Students
Courses
Assisting
TA/Graders

40. But what if students were divided into sections, each headed by a TA?
Works in CS180, because each TA (or grader) is a TA of all students. Connection student-TA is only via the course.
But what if students were divided into sections, each headed by a TA?
Then, a student in CS180 would be related to only one of the TA's for CS180. Which one?
Need a 3-way relationship to tell.

41. Students Courses TAs Ann CS180 Jan Sue CS180 Pat Bob CS180 Jan … … …
Enrolls
Students
TAs
Students Courses TAs
Ann CS180 Jan
Sue CS180 Pat
Bob CS180 Jan
… … …

42. Beers-Bars-Drinkers Example
Our running example for the course.
name
addr
license
Frequents
Serves
Bars
Likes
Beers
Drinkers
name
manf
name
addr

43. Multiplicity of Relationships
Many-many
Many-one
One-one
Representation of Many-One
E/R: arrow pointing to “one.”
Rounded arrow = “exactly one.”

44. Example: Drinkers Have Favorite Beers
name
addr
license
Frequents
Serves
Bars
Likes
Beers
Drinkers
Favorite
name
manf
name
addr

45. One-One Relationships
Put arrows in both directions.
Design Issue:
Is the rounded arrow justified?
Here, manufacturer is an E.S.
In earlier diagrams it is an attribute.
Which is right?
Best- seller
Manfs
Beers

46. Attributes on Relationships
price
Sells
Bars
Beers
Shorthand for 3-way relationship:
price
Prices
Sells
Bars
Beers

47. A true 3-way relationship.
Price depends jointly on beer and bar.
Notice arrow convention for multiway relationships: “all other E.S. determine one of these.”
Not sufficiently general to express any possibility.
However, if price, say, depended only on the beer, then we could use two 2-way relationships: price-beer and beer-bar.
Or better: just make price an attribute of beer.

48. Converting Multiway to 2-Way
Baroque in E/R, but necessary in certain “object-oriented” models.
Create a new connecting E.S. to represent rows of a relationship set.
E.g., (Joe's Bar, Bud, $2.50) for the Sells relationship.
Many-one relationships from the connecting E.S. to the others.
BBP
The- Bar
The- Beer
The- Price
Bars
Beers
Price

49. Roles Sometimes an E.S. participates more than once in a relationship.
Label edges with roles to distinguish.
Married
Husband Wife
d1 d2
d3 d4
… …
husband
wife
Drinkers

50. Notice Buddies is symmetric, Married not. Design Question
Buddy1 Buddy2
d1 d2
d1 d3
d2 d1
d2 d4
… …
Buddies 1 2
Drinkers
Notice Buddies is symmetric, Married not.
No way to say “symmetric” in E/R.
Design Question
Should we replace husband and wife by one relationship spouse?

51. More Design Issues 1. Subclasses. 2. Keys.
3. Weak entity sets. (Next class.)

52. Subclasses Subclass = special case = fewer entities = more properties.
Example: Ales are a kind of beer. In addition to the properties (= attributes and relationships) of beers, there is a “color” attribute for ales.

53. E/R Subclasses
Assume subclasses form a tree (no multiple inheritance).
isa triangles indicate the subclass relation.
Beers
name
manf
isa
Ales
color

54. Different Subclass Viewpoints
1. E/R viewpoint: An entity has a component in each entity set to which it logically belongs.
Its properties are the union of the properties of these E.S.
2. Contrasts with object-oriented viewpoint: An object (entity) belongs to exactly one class.
It inherits properties of its superclasses.
Beers
name
manf
isa
Pete’s Ale
Ales
color

55. Multiple Inheritance
Theoretically, an E.S. could be a subclass of several other entity sets.
name
manf
name
manf
Beers
Wines
isa
isa
Grape
Beers

56. Problems How should conflicts be resolved?
Example: manf means vintner for wines, bottler for beers. What does manf mean for “grape beers”?
Need ad-hoc notation to resolve meanings.
In practice, we shall assume a tree of entity sets connected by isa, with all “isas” pointing from child to parent.

57. Keys
A key is a set of attributes whose values can belong to at most one entity.
In E/R model, every E.S. must have a key.
It could have more than one key, but one set of attributes is the “designated” key.
In E/R diagrams, you should underline all attributes of the designated key.

58. Example Suppose name is key for Beers. Beer name is also key for ales.
In general, key at root is key for all.
Beers
name
manf
isa
Ales
color

59. Example: A Multiattribute Key
number
hours
Courses
dept
room
Possibly, the combination of hours + room also forms a key, but we have not designated it as such.