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The Relational Data Model Tables Schemas Conversion from E/R to Relations

A Relation is a Table ,[object Object],[object Object],[object Object],[object Object],Attributes (column headers) Tuples (rows)

Schemas ,[object Object],[object Object],[object Object],[object Object]

Why Relations? ,[object Object],[object Object],[object Object],[object Object]

From E/R Diagrams to Relations ,[object Object],[object Object],[object Object],[object Object]

Entity Set -> Relation ,[object Object],Beers name manf

Relationship -> Relation Drinkers Beers name addr name manf Likes Likes(drinker, beer) Favorite Favorite(drinker, beer) Married husband wife Married(husband, wife) Buddies 1 2 Buddies(name1, name2)

Combining Relations ,[object Object],[object Object]

Risk with Many-Many Relationships ,[object Object],name addr beer Sally 123 Maple Bud Sally 123 Maple Miller Redundancy

Handling Weak Entity Sets ,[object Object],[object Object]

Example Logins Hosts At name name Hosts(hostName) Logins(loginName, hostName, time) At(loginName, hostName, hostName2) time Must be the same At becomes part of Logins

Entity Sets With Subclasses ,[object Object],[object Object],[object Object],[object Object]

Example Beers Ales isa name manf color

E/R Style name manf Bud Anheuser-Busch Summerbrew Pete’s Beers name color Summerbrew dark Ales

Using Nulls name manf color Bud Anheuser-Busch NULL Summerbrew Pete’s dark Beers

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