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Examplesof

Defining

Constraints

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An ER diagram for a COURSES Database

A course may or may not usea textbook

A text by definition is a book

that is used in some course A course may not use more

than five books

Instructors teach from two tofour courses

Each course is taught by exactly one instructor

Each textbook is used by one and only one course

Add the relationship ADOPTS between INSTRUCTOR andTEXT

An instructor does not have to adopt a textbook for all courses

An instructor does not adopt more than three textbooks foreach course

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An ER diagram with defined Constraints

A course may or may not usea textbook.

A text by definition is a book

that is used in some course. A course may not use more

than five books.

Instructors teach from two tofour courses.

Each course is taught by exactly one instructor.

Each textbook is used by one and only one course.

Add the relationship ADOPTS between INSTRUCTOR andTEXT.

An instructor does not have to adopt a textbook for all courses.

An instructor does not adopt more than three textbooks foreach course.

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Another Scenario

There are musicians, identified by first and last name,and each plays an instrument.

There are bands, each with a unique band name. Each

band has a style. CDs are identified by their product ids, and each has a

title and publisher.

A musician is a member of one or more bands and aband has one or more musicians. For each musician ina band there is a date the musician joined and a datethe musician left.

A musician appears on one or more CDs.

Each CD is released by a single band, but a band canrelease any number of CDs. A CD is released by a bandon a release date.

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A possible ER diagram

(1,N)Musician Band CD

Member-of

Released

First

Last

Instrument

Bname

Style

Join-date

Release-date

PidTitle

Publisher

(1,N) (0,N) (0,1)

Find at least 5 problems with this ER diagram relativeto the scenario described above.

List them, and show on the diagram how to fix each

problem.

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The five problems with this ER diagram

(1,N)Musician Band CD

Member

-of

Released

First

Last

InstrumentBname

Style

Join-date

Release-date

Pid Title

Publisher

(1,N) (0,N) (0,1)

Instrument should not be part of the key of Musician.

Missing relationship for Musician appearing on CD.

No date of leaving on Member-of relationship.

Every CD is issued by a Band, so the minimum cardinality on theCD end of the relationship should be 1.

Release date should not be an attribute of Band, since a Band canrelease several CDs. It should be an attribute on either the

Released relationship or the CD entity.

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Modified ER diagram

(1,N)

Musician Band CDMember

-ofReleased

First

Last

InstrumentBname

Style

Join-date

Release-date

PidTitle

Publisher

(1,N) (0,N) (1,1)

1

Appears-

on

(1,N)(1,N)

5

2

4

3Leave-date