ch2_Relational_model_intro

ch2_Relational_model_intro - Chapter 2 Intro to Relational...

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Unformatted text preview: Chapter 2: Intro to Relational Model Chapter 2: Intro to Relational Model ©Silberschatz, Korth and Sudarshan 2.2 Database System Concepts - 6 th Edition Example of a Relation Example of a Relation attributes (or columns) tuples (or rows) ©Silberschatz, Korth and Sudarshan 2.3 Database System Concepts - 6 th Edition Attribute Types Attribute Types ■ 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 ■ The special value null is a member of every domain ■ The null value causes complications in the definition of many operations ©Silberschatz, Korth and Sudarshan 2.4 Database System Concepts - 6 th Edition Relation Schema and Instance Relation Schema and Instance ■ A 1 , A 2 , …, A n are attributes ■ R = ( A 1 , A 2 , …, A n ) is a relation schema Example: instructor = ( ID, name, dept_name, salary ) ■ 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 ■ The current values ( relation instance ) of a relation are specified by a table ■ An element t of r is a tuple , represented by a row in a table ©Silberschatz, Korth and Sudarshan 2.5 Database System Concepts - 6 th Edition Relations are Unordered Relations are Unordered ■ Order of tuples is irrelevant (tuples may be stored in an arbitrary order) ■ Example: instructor relation with unordered tuples ©Silberschatz, Korth and Sudarshan...
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This note was uploaded on 09/10/2011 for the course CS 308 taught by Professor Frankl during the Spring '11 term at NYU Poly.

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ch2_Relational_model_intro - Chapter 2 Intro to Relational...

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