√2011-09-02-RelationalAlgebra

√2011-09-02-RelationalAlgebra - Relational...

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Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Relational Algebra Chapter 4, Part A
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Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Relational Query Languages Query languages : Allow manipulation and retrieval of data from a database. Relational model supports simple, powerful QLs: Strong formal foundation based on logic. Allows for much optimization. Query Languages != programming languages! QLs not expected to be “Turing complete”. QLs not intended to be used for complex calculations. QLs support easy, efficient access to large data sets.
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Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Formal Relational Query Languages Two mathematical Query Languages form the basis for “real” languages (e.g. SQL), and for implementation: Relational Algebra : More operational , very useful for representing execution plans. Relational Calculus : Lets users describe what they want, rather than how to compute it. ( Non- operational, declarative .)
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Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4 Preliminaries A query is applied to relation instances , and the result of a query is also a relation instance. Schemas of input relations for a query are fixed (but query will run regardless of instance!) The schema for the result of a given query is also fixed! Determined by definition of query language constructs. Positional vs. named-field notation: Positional notation easier for formal definitions, named-field notation more readable. Both used in SQL
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Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 5 Example Instances R1 S1 S2 “Sailors” and “Reserves” relations for our examples. We’ll use positional or
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This note was uploaded on 12/07/2011 for the course CS 4410 taught by Professor Vollset during the Spring '07 term at Cornell University (Engineering School).

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√2011-09-02-RelationalAlgebra - Relational...

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