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# ch2 - Database System Concepts 5 th Ed ©Silberschatz Korth...

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Unformatted text preview: Database System Concepts, 5 th Ed. ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use Chapter 2: Relational Model Chapter 2: Relational Model ©Silberschatz, Korth and Sudarshan 2.<number> Database System Concepts - 5 th Edition, June 15, 2005 Chapter 2: Relational Model Chapter 2: Relational Model ■ Structure of Relational Databases ■ Fundamental Relational-Algebra-Operations ■ Additional Relational-Algebra-Operations ■ Extended Relational-Algebra-Operations ■ Null Values ■ Modification of the Database ©Silberschatz, Korth and Sudarshan 2.<number> Database System Concepts - 5 th Edition, June 15, 2005 Example of a Relation Example of a Relation ©Silberschatz, Korth and Sudarshan 2.<number> Database System Concepts - 5 th Edition, June 15, 2005 Basic Structure Basic Structure ■ Formally, given sets D 1 , D 2 , …. D n a relation r is a subset of D 1 x D 2 x … x D n Thus, a relation is a set of n-tuples ( a 1 , a 2 , …, a n ) where each a i ∈ D i ■ Example: If ● customer_name = {Jones, Smith, Curry, Lindsay, …} /* Set of all customer names */ ● customer_street = {Main, North, Park, …} /* set of all street names*/ ● customer_city = {Harrison, Rye, Pittsfield, …} /* set of all city names */ Then r = { (Jones, Main, Harrison), (Smith, North, Rye), (Curry, North, Rye), (Lindsay, Park, Pittsfield) } is a relation over customer_name x customer_street x customer_city ©Silberschatz, Korth and Sudarshan 2.<number> Database System Concepts - 5 th Edition, June 15, 2005 Attribute Types Attribute Types ■ Each attribute of a relation has a name ■ The set of allowed values for each attribute is called the domain of the attribute ■ Attribute values are (normally) required to be atomic ; that is, indivisible ● E.g. the value of an attribute can be an account number, but cannot be a set of account numbers ■ Domain is said to be atomic if all its members are atomic ■ The special value null is a member of every domain ■ The null value causes complications in the definition of many operations ● We shall ignore the effect of null values in our main presentation and consider their effect later ©Silberschatz, Korth and Sudarshan 2.<number> Database System Concepts - 5 th Edition, June 15, 2005 Relation Schema Relation Schema ■ A 1 , A 2 , …, A n are attributes ■ R = ( A 1 , A 2 , …, A n ) is a relation schema Example:...
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ch2 - Database System Concepts 5 th Ed ©Silberschatz Korth...

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