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Unformatted text preview: . . Fall 2007 CPE/CSC 365: Introduction to Database Systems Alexander Dekhtyar . . SQL: Structured Query Language SELECT Statement select Statement SQL DML consists, largely, of select statement. Basic select statement looks as follows: select [ distinct ] select-list from from-list [ where qualification ] • from-list contains the list of database relations from which the data is to be retrieved. • select-list contains the list of relation attributes (possibly modified) to be returned in the answer to the query. • qualification contains the the conditions which must be satisfied by a database record to be put into the answer set. Evaluation of select statement: 1. from-list defines a cartesian product of all relations in it. 2. qualification defines selection and join conditions on the data. 3. select-list defines the final look of the answer set (i.e., the projection at- tributes). 4. distinct specifies duplicate elimination in the final answer set (default for select query : no duplicate elimination). SQL select Statement and Relational Algebra As one can guess from its evaluation, select statement implements Relational Algebra operations selection , projection , cartesian product and join . 1 select and selection Relational algebra operation: σ C ( R ) select statement select * from R where C Note: “*” is a special notation for the selection list that includes all available attributes ....
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This note was uploaded on 05/19/2008 for the course CSC 365 taught by Professor Dekhtyar during the Spring '08 term at Cal Poly.
- Spring '08