Ch4_Algebra-95 - Database Management Systems 3ed, R....

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Unformatted text preview: Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Relational Algebra Chapter 4, Part A Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 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. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 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 .) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 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 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 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. Well use positional or named field notation, assume that names of fields in query results are `inherited from names of fields in query input relations. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Relational Algebra Basic operations: Selection ( ) Selects a subset of rows from relation....
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Ch4_Algebra-95 - Database Management Systems 3ed, R....

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