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The Relational Data Model

The Relational Data Model. Tables Schemas Conversion from E/R to Relations. Attributes (column headers). Tuples (rows). A Relation is a Table. name manf Winterbrew Pete’s Bud Lite Anheuser-Busch Beers. Schemas. Relation schema = relation name and attribute list.

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The Relational Data Model

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

  2. Attributes (column headers) Tuples (rows) A Relation is a Table name manf Winterbrew Pete’s Bud Lite Anheuser-Busch Beers

  3. 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.

  4. Why Relations? • Very simple model. • Often matches how we think about data. • Abstract model that underlies SQL, the most important database language today.

  5. 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.

  6. Entity Set -> Relation Relation: Beers(name, manf) name manf Beers

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

  8. Combining Relations • OK to combine into one relation: • The relation for an entity-set E • The relations for many-one relationships of which E is the “many.” • Example: Drinkers(name, addr) and Favorite(drinker, beer) combine to make Drinker1(name, addr, favBeer).

  9. Redundancy Risk with Many-Many Relationships • Combining Drinkers with Likes would be a mistake. It leads to redundancy, as: name addr beer Sally 123 Maple Bud Sally 123 Maple Miller

  10. 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).

  11. Must be the same At becomes part of Logins Example name name Logins At Hosts location billTo Hosts(hostName, location) Logins(loginName, hostName, billTo) At(loginName, hostName, hostName2)

  12. Subclasses: Three Approaches • Object-oriented: One relation per subset of subclasses, with all relevant attributes. • Use nulls: One relation; entities have NULL in attributes that don’t belong to them. • E/R style: One relation for each subclass: • Key attribute(s). • Attributes of that subclass.

  13. Example Beers name manf isa Ales color

  14. Object-Oriented name manf Bud Anheuser-Busch Beers name manf color Summerbrew Pete’s dark Ales Good for queries like “find the color of ales made by Pete’s.”

  15. E/R Style name manf Bud Anheuser-Busch Summerbrew Pete’s Beers name color Summerbrew dark Ales Good for queries like “find all beers (including ales) made by Pete’s.”

  16. Using Nulls name manf color Bud Anheuser-Busch NULL Summerbrew Pete’s dark Beers Saves space unless there are lots of attributes that are usually NULL.

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