christoph f. eick: designing e/r diagrams 1 designing e/r diagrams updated: january 25. 2005

Christoph F. Eick: Designing E/R Diagrams 1

Designing E/R Diagrams

Updated: January 25. 2005


Conceptual Data Models

Conceptual data models provide languages to describe conceptual schemas.

Conceptual schemas are used to describe the classes of objects that occur in an application area, their properties, their relationships, and the constraints that hold with respect to those classes of objects.

Center on “what kind of objects a database contains” and not on “how these objects are stored” ( Internal Schema) and not on “how these objects are represented / displayed to a person that accesses the database” ( External Schema).


Conceptual Data Models ---What are they good for?

As a database design tool formalizing the information requirements of the end users

As a documentation tool for databases (to help programmers, especially those that have to update the database)

As a data model of a database management system (only very few experimental systems exist)

As a tool to describe domain ontologies (terminology and concepts in a UoD)

As a tool of system analysis

X

X

Person

name

ssn

Wedding

E/R Model Symbols for COSC 3480

occurred

to

Is-insured

T1

amount

(n,m)

phone#

from-------

Entity Type

Attribute

RelationshipType

Weak Entity Type

IdentifyingRelationship

Key Attribute

Partial Key

Optional Attribute

R

Entities of type X participate at least n, at most mtimes in relationship R; * indicates .

Y

Entity type X is a subtype of type Y

T2

Type T1 and T2 are overlapping; an entity canbelong to both T1 and T2; default is disjoint

Multi-valued Attribute

Derived Attribute

T2

T3

T1

T3=T2 T1

Course

Student

Department

name

ssn

grade

took

semesterid title

salary

(0,30)

(0,*)

(1,1)

University Problem Final’03 COSC 3480

employs

Cou#

Semester

took-not-a-set: -2.5Other Errors: -1.5 (or 3-4 if major)

Grad

UGrad

gre

(0,*)

(0,*)

mentor

name

(0,25)

(0,2)

home

(0,*)

(1,1))


Exam1 Fall 2005 Problem Design a “good” entity-relationship diagram that describes the following

objects in a university application: students, departments, sections taught in the present and future, and courses. Departments have a name that uniquely identifies the department. Students are identified by a unique social security number, zero, one or multiple e-mail addresses, and an optional gpa (new students do not have a gpa yet). Courses have a unique course number and a course title. Courses are offered in one or more sections at a particular time. Sections are identified by the time they are offered (e.g. 10:30-noon TUTH) and by the course they are associated with. Additionally sections are characterized by the class room the section is taught in. Only information concerning sections that are taught in the present or in the future is stored in the database. Students take a course in a particular semester and receive a grade for their performance. Sometimes students take the same course again in a different semester. There are no limits on how many courses a student can complete, and on how many students completed a particular course. Each student is associated with a least one department. Some students are graduate students that are additionally characterized by their most recent GRE-score. Some graduate students work for a department and receive a salary for their services. Each department employs at most 75 graduate students; graduate students are not allowed to work for multiple departments.

Course

Student

Department

name

ssn

grade

took

semesterid

title

salary

(0,75)

(0,*)

(0.*)

University Problem Exam1’05

employs

Cou#

Semester

took-not-a-set: -1.5Section not weak: -2Other Errors: -0.5--1 if minor -2—3 if major

Grad

E-mail

(0,*)

(0,*)

assoc

name

(0,1)

(0,*)

(1,1)

gpa(1,*)

gre

Section

offered

Time-offered classroom


Team contr

name

cityPlayer

namessn

birthd

played-in.

Date

pos

score

Home Visit

Sal

(1,1)

(25,*)

(18,*)

(0,*)(0,*)

Solution Problem6 Exam0 Spring 2003

(0,*)

play

Game#

Game

Contract set viol –3, Other: -2

pos

Month

fromto(0,*)

(0,*)

(0,*)

name


Team

empl.

contr

name

city

Manager

Player

Person

namessn

weight pos

birthd

height

Game

play played-in.

Date

minscore

Home Visit

Sal

Sal

(1,3)

(0,4)

(1,1)

(24,99) (0,1)

(22,*)

(0,*)(0,*)NFLE/RProblem

(0,*)

Scoring: 1. Play relationship a Set: 32. Person/Player/Manager: 33. Weak Game Entity: 34. Played-in: 25. Can Only Play once on a day: 16. Contract: 37. Salary, score, min attribute: 3


Identifying Keys and Relationships

for Entity TypesEach entity type that is not subtype of another entity

type needs: Case1: Normal Entity Type (single rectangle)

1. A single attribute (straight line) or2. A set of attributes or

Case2: Weak Entity Type (double rectangle)1. A set of relationships (double diamond) or2. A set of relationships or a single attribute (dotted line) or3. A set of relationships and a set of attributes (dotted line)

that uniquely identifies the instances of the entity type

Remark: min-max cardinalities for weak entity types for their participation in identifying relationships have to be (1,1)!!

Male

Female

Person

namessn

Wedding

(0,*)

E/R Diagram for Multi-Weddings

occurred

(0,*)

fromto

(1,1)

Is-insured Company

nameamount

(0,*)(0,*)

location

husband

wife


Valid E/R Diagrams

An E/R diagram is valid if and only if: It is syntactically correct (e.g.

specifies all key constraints,…) It specifies the entity types, relationship

types, attribute types, and subtype relationships necessary to satisfy all information requirements.

It does not specify any invalid constraints.


Priorities when Choosing Between Valid E/R Diagrams

1. Express all constraints (you can express!)

2. Use and do not change terminology and class structure of the application domain

3. Keep it simple (avoid defining entity types that do not serve any purpose)

4. Avoid redundancy (but derived attributes are okay)!


A Quite Bad E/R Diagrams

Company

ssn

gender

works-for

departmentgpa

(0,*)

Name

Person

salary

(0,*)(0,*)

is-married-to

husbandwife

(0,*) takes

Section

(0,*)

Course

(0,*)

C#

S#

time


Example: Too many Entity Types /Don’t use Foreign Keys

Company

ssn

name

is-insured

Name

Person(0,*)(0,*)

Example: Persons as well as animals can be insured

Animal

P#

(0,*)

A#

Boss-ssn

Bad E/R Diagram!


E/R Diagram Design – Typical Errors

1. Missing Constraints2. Unexpressed Constraints due to bad design3. Every entity type needs a key4. Attribute associated with the wrong entity type (relationship type)5. Relationships are sets!6. No partial participation in relationships!7. Missing existence dependencies (use subclasses)8. Invalid constraints9. Using Subtypes for n:1 relationships; using relationships when subtypes should

be used.10. When defining relationships: Too general entity types for participating entities 11. Too many entity types12. Using foreign keys instead of relationships


Other Issues in E/R Design

1. No relationships of relationships --- solution: create an entity type that represent instances of the relationship (or use aggregation as discussed in the textbook)

2. value or entity type --- solution: choose entity type if it helps expressing constraints; otherwise, use value-type.


University E/R Design Problem

Design a “good” entity-relationship diagram that describes the following objects in an university application: students, instructors, professors, and courses. Students are subdivided into graduate and undergraduate students. Students take a course in a particular semester and receive a grade for their performance. Sometimes students take the same course again in a different semester. There are no limits on how many courses a student can take, and on how many students completed a particular course. Each graduate student has exactly one advisor, who must be a professor, whereas each professor is allowed to be the advisor of at most 20 students. Courses have a unique course number and a course title. Students and professors have a name and a unique ssn; students additionally have a gpa; moreover, graduate students have a GRE-score, and undergraduate students have a single or multiple majors. Professors can be students and take courses, but graduate students cannot be undergraduate students.

Indicate the cardinalities for each relationship type; assign roles (role names) to each relationship if there are ambiguities! Use sub-types, if helpful to express constraints!

Course

Student

Professor

Person

namessn

gradegpa

took

semesteridtitle

gre(0,1)

(0,*)

(0,20)

University Problem (slightly different from Exam0’03)

advises

Cou#

Semester

Enrolls-not-a-set: -4Student must be ugrad or grad: -1Other Errors: -2 (or –3 if quite major)

Ugrad

Grad

major

(0,*)

(0,*)

Hotel

Gold_Cl.

Client

addr

ssn

discount

G#

room-type

address

number

(0,*)

A:=guaranteed; B:=has_transaction; C:=for_hotel; D:=consists_of; E:=for_category; F=avail-rooms; G=total-rooms; modified on Feb. 3, 2004

Ho#

Transaction

Category

Cred-Card

(0,*)

(0,1)

Reservation

company

C

E

D

(1,1)

B

(1,50)

(1,1)

(0,300)

(1,1)

F

#avail

from rateto

day_made

Res#

(1,1)

tr#

(0,*)

(0,*)

(1,*)

phone#

A

Problem 1

Exam1 Fall’03

Grading:Minor Error: -1Medium Error: -2Major Error: -3 or –40-4 points if too many errors

day

Date#totalG

(0,*)

(1,*)


Aggregation Used when we

have to model a relationship involving (entity sets and) a relationship set. Aggregation

allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships.

Aggregation vs. ternary relationship: Monitors is a distinct relationship, with a descriptive attribute. Also, can say that each sponsorship is monitored by at most one employee.

budgetdidpid

started_on

pbudgetdname

until

DepartmentsProjects Sponsors

Employees

Monitors

lotname

ssn

since


NFL E/R Design ---Ungraded Homework --- due: Th., Jan.

27,2005

Design an Entity-Relationship Diagram that models the following objects and relationships in the world of football (NFL): teams, players, games, managers and contracts. Each (NFL-) team has a unique team name, and a city it plays in. Each person being part of the NFL-world has a unique ssn and a name. Additionally, for players their weight, height, position and birth dates are of importance. Players have a contract with at most one team and receive a salary for their services, and teams have at least 24 and at most 99 players under contract. Each team has one to three managers; managers can work for at most 4 teams and receive a salary for each of their employments. Players cannot be managers. A game involves a home-team and visiting-team; additionally, the day of the game, and the score of the game are of importance; teams play each other several times in a season (not on the same day!). Moreover, for each game played we like to know which players participated in the game and how many minutes they played.

Indicate the cardinalities for each relationship type; assign roles (role names) to each

relationship if there are ambiguities! Use sub-types, if helpful to express constraints!

christoph f. eick: designing e/r diagrams 1 designing e/r diagrams updated: january 25. 2005

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