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Announcement• Recitation time

Before midterm: 6-7pm, by Earl Wagner After midterm: 5-6pm, by Yi Qiao

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mailing list Send all course related questions there for

timely response (unless privacy needed)

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The Relational Data Model

TablesSchemas

Conversion from E/R to Relations

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A Relation is a Table

name manfWinterbrew Pete’sBud Lite Anheuser-

BuschBeers

Attributes(columnheaders)

Tuples(rows)

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Schemas

• Relation schema = relation name and attribute list. Optionally: types of attributes. Example: Beers(name, manf) or

Beers(name: string, manf: string)• Database = collection of relations.• Database schema = set of all

relation schemas in the database.

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Why Relations?

• Very simple model.• Often matches how we think about

data.• Abstract model that underlies SQL,

the most important database language today.

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From E/R Diagrams to Relations

• Entity set -> relation. Attributes -> attributes.

• Relationships -> relations whose attributes are only: The keys of the connected entity sets. Attributes of the relationship itself.

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Entity Set -> Relation

Relation: Beers(name, manf)

Beers

name manf

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Relationship -> Relation

Drinkers BeersLikes

Likes(drinker, beer)Favorite

Favorite(drinker, beer)

Married

husband

wife

Married(husband, wife)

name addr name manf

Buddies

1 2

Buddies(name1, name2)

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Combining Relations

• OK to combine into one relation:1. The relation for an entity-set E 2. The relations for many-one

relationships from E (“many”) to F

• Example: Drinkers(name, addr) and Favorite(drinker, beer) combine to make Drinker1(name, addr, favBeer).

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Combining Relations (II)

• The combined relation schema consists of All attributes of E The key attributes of F Any attributes belonging to the

relationship R

• Can we combine one-one relationship?

• What about many-many?

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Risk with Many-Many Relationships

• Combining Drinkers with Likes would be a mistake. It leads to redundancy, as:

name addr beerSally 123 Maple BudSally 123 Maple Miller

Redundancy

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Handling Weak Entity Sets

• Relation for a weak entity set must include attributes for its complete key (including those belonging to other entity sets), as well as its own, nonkey attributes.

• A supporting relationship is redundant and yields no relation.

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Example

Logins HostsAt

name name

Hosts(hostName, location)Logins(loginName, hostName, billTo)At(loginName, hostName, hostName2)

Must be the same

billTo

At becomes part ofLogins

location

What if “At” has some attributes ?

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Case Study

cities

counties states

Popu. name

Located

Co. Popu. Co. name

capitals

Ci. Popu. Ci. name

Belongs-to

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Sample Solution• States (name, popu)• Conuties (co name, state name, co

popu)• Cities (ci name, co name, state

name, ci popu)• Capitals (state name, ci name, co

name)

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Subclasses: Three Approaches

1. Object-oriented : One relation per subset of subclasses, with all relevant attributes.

2. Use nulls : One relation; entities have NULL in attributes that don’t belong to them.

3. E/R style : One relation for each subclass: Key attribute(s). Attributes of that subclass.

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Example

Beers

Ales

isa

name manf

color

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Object-Oriented

name manfBud Anheuser-Busch

Beers

name manf colorSummerbrew Pete’s dark

Ales

Good for queries like “find thecolor of ales made by Pete’s.”

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E/R Stylename manfBud Anheuser-BuschSummerbrew Pete’s

Beers

name colorSummerbrew dark

Ales

Good for queries like“find all beers (includingales) made by Pete’s.”

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Using Nulls

name manf colorBud Anheuser-Busch NULLSummerbrew Pete’s dark

Beers

Saves space unless there are lotsof attributes that are usually NULL.

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Isa

staff faculty student assistant

Case Study

employee

ssno

salary

name

position rank PercentageTime

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Relations:

employee(ssno, name, salary)

staff(ssno, name, salary,position)

faculty(ssno, name, salary, rank)

studentassistant(ssno, name, salary, percentagetime)

Key: ssno for all the relations

Isa

staff faculty Student assistant

employee

ssno

salary

name

position rank Time percentage

Subclass – Object-oriented

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Relations: employee(ssno, name, salary)staff(ssno, position) faculty(ssno, rank) studentassistant(ssno, percentage_time)

Key: ssno for all relations

Isa

staff faculty student assistant

Subclass – E/R Style

employee

ssno

salary

name

position rank PercentageTime

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Isa

staff faculty Student assistant

employee

ssno

salary

name

position rank PercentageTime

Relation:

employee(ssno, name, salary, position, rank, percentage-time)

Key : ssno as key

Note: Sometimes we add an attribute “jobType” to make queries easier.

Subclass – null value