Understanding Relational Data Model: Entity Sets to Relations and Table Combination, Slides of Database Management Systems (DBMS)

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The Relational Data Model

A Relation is a Table

name manf

Snickers M&M/Mars

Twizzlers Hershey

Candies

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Attributes (column headers)

Tuples (rows)

Why Relations?

• Very simple model.

• Often matches how we think about data.

• Abstract model that underlies SQL, the most

important database language today.

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.

Relationship -> Relation

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Con- sumers Likes Candies

Likes(consumer, candy)

Favorite

Favorite(consumer, candy)

Married

husband

wife

Married(husband, wife)

name addr name manf

Buddies

Buddies(name1, name2)

Combining Relations

• OK to combine into one relation:

1. The relation for an entity-set E

2. The relations for many-one relationships of

which E is the “many.”

• Example: Consumers(name, addr) and

Favorite(consumer, candy) combine to make

Consumer1(name, addr, favCandy).

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 (unless it has attributes).

Example

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Logins At Hosts

name name

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

Must be the same

billTo

At becomes part of Logins

location

Example

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Candies

Choc- olates

isa

name (^) manf

color

Object-Oriented

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name manf Twizzler Hershey Candies

name manf color Snickers M&M/Mars light Chocolates

Good for queries like “find the color of chocolate candies made by M&M/Mars.”

Using Nulls

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name manf color Twizzler Hershey NULL Snickers M&M/Mars dark Candies

Saves space unless there are lots

of attributes that are usually NULL.