CS 450 Relational Algebra 1 - George Mason University
CS 450 Relational Algebra 1
Prof. Jessica Lin
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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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. (Nonoperational, declarative.)
Understanding Algebra is key to understanding SQL,
and query processing!
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The Role of Relational Algebra in a DBMS
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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.
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