Data Modeling Entity Relationship Diagrams

5/27/2018 Data Modeling Entity Relationship Diagrams

1/27Dale Roberts 1

Department of Computer and Information Science,

School of Science, IUPUI

Dale Roberts, Lecturer

Computer Science, IUPUI

E-mail: [email protected]

Data Modeling

Entity-Relationship Diagrams


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Framework for E-R

Design is a serious business.

The boss knows they want a database, butthey dont know what they want in it.

Sketching the key components is an efficient

way to develop a working database.


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Purpose of E-R Model

The E-R model allows us to sketch

database schema designs.Includes some constraints, but not operations.

Designs are pictures called ent i ty-

relat ionsh ip diagrams.Later: convert E-R designs to relational

DB designs.


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Entity Sets

Enti ty= thing or object.

Enti ty set= collection of similar entities.Similar to a class in object-oriented languages.

Att r ibute= property of (the entities of) an

entity set.Attributes are simple values, e.g. integers or character

strings, not structs, sets, etc.


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E-R Diagrams

In an entity-relationship diagram:

Entity set = rectangle.

Attribute = oval, with a line to the rectangle representing

its entity set.

Warning!

Graphical representations are inconsistent.

Attributes using ovals shall be changed to more of a

Class Diagram style later, to save space.

The later style insists on singular nouns and uses lines for

relationships.


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Example: Entity

Entity set Beershas two attributes, nameand

manf(manufacturer).

Each Beersentity has values for these twoattributes, e.g. (Bud, Anheuser-Busch)

Underlined attribute(s) is(are) primary key(s)

Beers

name manf


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Relationships

This relationship has an attribute associated

with it.

A relationshipconnects two or more entity sets.

It is represented by a diamond, with lines toeach of the entity sets involved.


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Example: Relationships

Drinkers addrname

Beers

manfname

Bars

name

license

addr

Note:license =

beer, full,none

Sells Bars sell somebeers.

Likes

Drinkers likesome beers.Frequents

Drinkers frequentsome bars.


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Relationship Set

The current value of an entity set is the set of

entities that belong to it.Example: the set of all bars in our database.

The value of a relationship is a relat ionship

set, a set of tuples with one component for each

related entity set.


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Example: Relationship Set

For the relationship Sells, we might have a

relationship set like:

Bar BeerJoes Bar BudJoes Bar MillerSues Bar BudSues Bar Petes AleSues Bar Bud Lite



Multiway Relationships

Sometimes, we need a relationship that

connects more than two entity sets.Suppose that drinkers will only drink certain

beers at certain bars.

Our three binary relationships Likes, Sells, and Frequents

do not allow us to make this distinction.

But a 3-way relationship would.


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Example: 3-Way Relationship

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Bars Beers

Drinkers

namename

addrmanf

name addr

license

Preferences

Example: 3-Way Relationship



A Typical Relationship Set

Bar Drinker BeerJoes Bar Ann Miller

Sues Bar Ann BudSues Bar Ann Petes AleJoes Bar Bob BudJoes Bar Bob Miller

Joes Bar Cal MillerSues Bar Cal Bud Lite



Many-Many Relationships

Focus: binaryrelationships, such as Sells

between Bars and Beers.In a many-many relationship, an entity of either

set can be connected to many entities of the

other set.

E.g., a bar sells many beers; a beer is sold by many bars.



In Pictures:

many-many


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Many-One Relationships

Some binary relationships are many-onefrom

one entity set to another.Each entity of the first set is connected to at

most one entity of the second set.

But an entity of the second set can beconnected to zero, one, or many entities of the

first set.


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Dale Roberts 18

In Pictures:

many-one


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Dale Roberts 19

Example: Many-One Relationship

Favorite, from Drinkersto Beersis many-one.

A drinker has at most one favorite beer.

But a beer can be the favorite of any number of

drinkers, including zero.


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Example: Many-One Relationship

Drinkers BeersLikes

FavoriteNotice: two relationshipsconnect the same entitysets, but are different.


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One-One Relationships

In a one-onerelationship, each entity of either

entity set is related to at most one entity of theother set.

Example: Relationship Best-sellerbetween

entity sets Manfs(manufacturer) and Beers.A beer cannot be made by more than one manufacturer,and no manufacturer can have more than one best-

seller (assume no ties).


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In Pictures:

one-one


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Representing Multiplicity

Show a many-one relationship by an arrow

entering the one side.Remember: Like a functional dependency.

Show a one-one relationship by arrows

entering both entity sets.

Rounded arrow= exactly one, i.e., each

entity of the first set is related to exactly

one entity of the target set.

Pointed arrow= zero or one, i.e., eachentity of the first set is related to zero or

one entity of the target set. (Optionality)


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Example: One-One Relationship

Consider Best-sellerbetween Manfs and Beers.

Some beers are not the best-seller of anymanufacturer, so a rounded arrow to Manfs

would be inappropriate.

But a beer manufacturer has to have a best-seller.


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In the E-R Diagram

Manfs BeersBest-seller

A manufacturer hasexactly one best

seller.

A beer is the best-seller for 0 or 1manufacturer.


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Dale Roberts

Modeling Shift

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Dale Roberts3/31/2014

Acknowledgements

McFadden and Hoffer. Database Management

Loney, Kevin. Oracle Database 10g The CompleteReference

Ullman, Jeff. Database Systems The Complete Book.

DatabaseAnswers.com

Data Modeling Entity Relationship Diagrams

Documents