Algebra (1) - CSCD343 Introduction to databases A Vaisman...

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Unformatted text preview: CSCD343- Introduction to databases- A. Vaisman Relational Algebra CSCD343- Introduction to databases- A. Vaisman 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. CSCD343- Introduction to databases- A. Vaisman Formal Relational Query Languages Two mathematical Query Languages form the basis for “real” languages (e.g. SQL), and for implementation: Relational Algebra : More operational(procedural) , very useful for representing execution plans. Relational Calculus : Lets users describe what they want, rather than how to compute it. ( Non-operational, declarative .) CSCD343- Introduction to databases- A. Vaisman 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 CSCD343- Introduction to databases- A. Vaisman 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. “bid”= boats. “sid”: sailors 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. CSCD343- Introduction to databases- A. Vaisman Relational Algebra Basic operations: Selection ( ) Selects a subset of rows from relation....
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Algebra (1) - CSCD343 Introduction to databases A Vaisman...

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