lecture4 - INFS614-001, GMU, Dr. Brodsky, Lecture 4...

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Unformatted text preview: INFS614-001, GMU, Dr. Brodsky, Lecture 4 Relational Algebra Lecture 4 Professor Alex Brodsky INFS614 - Database Management INFS614-001, GMU, Dr. Brodsky, Lecture 4 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. INFS614-001, GMU, Dr. Brodsky, Lecture 4 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 Algebra is key to understanding SQL, and query processing! INFS614-001, GMU, Dr. Brodsky, Lecture 4 Algebra 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. INFS614-001, GMU, Dr. Brodsky, Lecture 4 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. INFS614-001, GMU, Dr. Brodsky, Lecture 4 Algebra Operations Look what we want to get from the following table: sid sname rating age 28 yuppy 9 35.0 31 lubber 8 55.5 44 guppy 5 35.0 58 rusty 10 35.0 INFS614-001, GMU, Dr. Brodsky, Lecture 4 Projection sname rating yuppy 9 lubber 8 guppy 5 rusty 10 sname rating S , ( ) 2 age 35.0 55.5 age S ( ) 2 Deletes attributes that are not in projection list . Schema of result contains exactly the fields in the projection list, with the same names that they had in the (only) input relation....
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This note was uploaded on 01/20/2010 for the course CS 614 taught by Professor Jessica during the Fall '09 term at George Mason.

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lecture4 - INFS614-001, GMU, Dr. Brodsky, Lecture 4...

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