数据库课件lec4algebra幻灯片.pptVIP

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Relational Algebra Jianlin Feng School of Software SUN YAT-SEN UNIVERSITY courtesy of Joe Hellerstein for some slides Relational Query Languages Query languages: manipulation and retrieval of data 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. (Actually, I no longer believe this. But it’s the standard viewpoint) Formal Relational Query Languages Relational Algebra: More operational, very useful for representing execution plans. Relational Calculus: Describe what you want, rather than how to compute it. (Non-procedural, declarative.) Understanding Algebra Calculus is key to understanding SQL, query processing! Preliminaries A query is applied to relation instances The result of a query is also a relation instance. Schemas of input relations for a query are fixed Schema for the result of a query is also fixed. determined by the query language constructs Positional vs. named-field notation: Positional notation easier for formal definitions Named-field notation more readable. Both used in SQL Though positional notation is discouraged Relational Algebra: 5 Basic Operations Selection ( s ) Selects a subset of rows (horizontal) Projection ( p ) Retains only desired columns (vertical) Cross-product ( ? ) Allows us to combine two relations. Set-difference ( — ) Tuples in r1, but not in r2. Union ( ? ) Tuples in r1 or in r2. Since each operation returns a relation, operations can be composed! (Algebra is “closed”.) R1 S1 S2 Boats Example Instances Projection (?) Example: Retains only attributes that are in the “projection list”. Schema of result: the fields in the projection list w