Transcription of Relational Algebra and SQL - Cornell University
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Relational Algebra and SQL - Cornell University
Database Management Systems, R. Ramakrishnan and J. Gehrke1 Relational Algebra and SQLJ ohannes fromDatabase Management Systems, 3rdEdition,Ramakrishnan and Management Systems, R. Ramakrishnan and J. Gehrke2 Relational Query LanguagesvQuery languages: Allow manipulation and retrieval of data from a model supports simple, powerful QLs: Strong formal foundation based on logic. Allows for much 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 Management Systems, R. Ramakrishnan and J. Gehrke3 Formal Relational Query LanguagesvTwo mathematical Query Languages form the basis for real languages ( 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.)Database Management Systems, R.
Relational Algebra vBasic operations: – Selection ( ) Selects a subset of rows from relation. – Projection ( ) Deletes unwanted columns from relation. – Cross-product ( ) Allows us to combine two relations. – Set-difference ( ) Tuples in reln. 1, but not in …
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