relationalAlgebra

# relationalAlgebra - Relational Algebra 1 Relational Query...

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1 Relational Algebra

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2 Relational Query Languages Query languages : Allow manipulation and retrieval of data from a database. Relational model supports simple, powerful query languages: 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.
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 .)      understanding SQL, query processing!

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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 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
5 Example Instances sid sname rating age 22 dustin 7 45.0 31 lubber 8 55.5 58 rusty 10 35.0 sid sname rating age 28 yuppy 9 35.0 31 lubber 8 55.5 44 guppy 5 35.0 58 rusty 10 35.0 sid bid day 22 101 10/10/96 58 103 11/12/96 R1 S1 S2 Sailors” and “Reserves” relations for our examples. We’ll use positional or named field notation, assume that names of fields in query results are `inherited’ from names of fields in query input relations.

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6 Relational Algebra Basic operations: Selection ( ) Selects a subset of rows from relation. Projection
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## This note was uploaded on 01/12/2011 for the course MAT 116 A MAT 116 A taught by Professor Hawkins during the Spring '10 term at University of Phoenix.

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relationalAlgebra - Relational Algebra 1 Relational Query...

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