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					ER-Diagram Examples

Ex. 1 - A Movie Domain
Domain Description: • A movie has a title, a year and a length. Since some movies have the same title, it takes a title and a year to uniquely identify a movie. • Some movies are remakes of others. • A star has a name and an address. A star's name uniquely identifies the star. • A star can appear in any number of movies.

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A Movie Domain
• Some movies have many stars and some have none. • A studio has a name and an address, and is uniquely identified by its name. • A star can belong to at most one studio. • A studio can own any number of movies. • A movie is always owned by at most one studio, but some are not owned by any studio.

A Movie Domain
• A studio may or may not have a president, but nobody can be a studio president without being the president of some studio. • Studio presidents are uniquely identified by their name, but they also have an address. • No one can be the president of more than one studio. • Stars and studio presidents are both examples of movie people, but there are other types of movie people as well. • No star can be a studio president.

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A Movie Domain
From the domain description we derive the entity types and their attributes (including primary keys): • Entity types and their attributes:
– Movie
• title, year, length

– Star
• name, address

– Studio
• name, address

– Studio President
• name, address

– Movie People
• name, address

A Movie Domain
From the domain description we derive the relationships between the entity types and their cardinalities: • Relationships:
– some movies are remakes of (exactly 1) movie; each movie has (0 or more) remakes – studios own (0 or more) movies; each movie is owned by (at most 1) studio – stars appear in (0 or more) movies; movies have (0 or more) stars appearing in them

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A Movie Domain
– stars belong to (at most 1) studio; studios have (0 or more) stars belonging to them – studio presidents are presidents of (exactly 1) studio; studios have (at most 1) president – stars and studio presidents are (disjoint but incomplete) subtypes of movie people

ER-Diagram for the Movie Domain
• Once we have the entity types, the attributes and the relationships, we can produce the ER-Diagram for the domain. • The ER-Diagram for the Movie Domain is shown on the next slide.

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remake-of

m o vie ow ns title y ear length

s tu d io name addres s

appears -in

m o vie -p e o p le name addres s

pres ident-of

D

s tar

s tu d io -p r e s

belongs -to

Ex. 2 - A Movie Domain
Domain Description: • Add to example (1) that a contract is a four-way relationship between a star, a movie, the star's studio and the studio that is producing the movie. • Also add that a contract specifies the star's salary for appearing in that movie.

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Movie Domain 2
• New entity types and their attributes: – Contract
• salary, star, movie title, movie year, star’s studio, producing studio

• Contract is an associative entity type used to represent a multi-way relationship. It is therefore a weak-entity type. All of the components of its primary key come from identifying relationships.

Movie Domain 2
• New relationships (these are all identifying relationships): – a contract has (exactly 1) star; a star can have (0 or more) contracts – a contract has (exactly 1) movie; a movie can have (0 or more) contracts – a contract has (exactly 1) star’s studio; a star’s studio can have (0 or more) contracts – a contract has (exactly 1) producing studio; a producing studio can have (0 or more) contracts

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ER-Diagram for Movie Domain 2
• The ER-Diagram for Movie Domain 2 is shown on the next slide.

remake-of

m ovie title year length movie-of -contract

s tudio ow ns studio-of -star name address

producing-studio contr act

salary appears-in m ovie -pe ople name address president-of

star-of -contract

D s tar s tudio-pre s

belongs-to

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posted:9/26/2009
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