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In-Depth Information
ORCHESTRA
PK
MUSICIAN
DEGREE
Orchestra
Name
PK
Musician
Number
PK
Musician
Number
PK
Degree
Employs
Employed by
Earned
City
Country
Music
Director
Musician
Name
Instrument
Annual
Salary
Earned by
University
Year
Recorded
Recorded by
COMPOSITION
PK
RECORDING
PK
COMPOSER
PK
Composition
Name
Orchestra
Name
Composer
Name
PK
Composition
Name
PK
Composer
Name
Recorded
Contains
Wrote
Written by
Country
Date of Birth
PK
Composer
Name
Year
Year
Price
F IGURE 2.11
World Music Association entity-relationship diagram
(but we still want to maintain data about it) or may have been recorded by many
orchestras. For a particular recording, we note the year of the recording and the retail
price, as intersection data of the many-to-many relationship between orchestras and
compositions. In fact, RECORDING is an associative entity. A composer may have
several compositions to his credit but must have at least one to be of interest to
WMA. A composition is associated with exactly one composer. COMPOSITION
is a dependent entity to COMPOSER, which means that the unique identifier of
COMPOSITION is the combination of Composer Name and Composition Name.
After all, there could be Beethoven's ''Third Symphony'' and Mozart's ''Third
Symphony.'' This has an important implication for the RECORDING associative
entity. To uniquely identify a recording (and attach the year and price intersection
data to it) requires an Orchestra Name, Composition Name, and Composer Name.
EXAMPLE: LUCKY RENT-A-CAR
Lucky Rent-A-Car's business environment is, obviously, centered on its cars. This
is literally true in its E-R diagram, shown in Figure 2.12. A car was manufactured by
exactly one manufacturer. A manufacturer manufactured at least one and generally
many of Lucky's cars. A car has had many maintenance events (but a brand new
car may not have had any, yet.) A car may not have been rented to any customers
(again, the case of a brand new car) or to many customers. A customer may have
rented many cars from Lucky, and to be in Lucky's business environment must
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