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Relational Algebra - Stanford University

Relational Algebra Basic Operations Algebra of Bags 1. What is an Algebra . Mathematical system consisting of: Operands --- variables or values from which new values can be constructed. Operators --- symbols denoting procedures that construct new values from given values. 2. What is Relational Algebra ? An Algebra whose operands are relations or variables that represent relations. Operators are designed to do the most common things that we need to do with relations in a database . The result is an Algebra that can be used as a query language for relations. 3. Core Relational Algebra Union, intersection, and difference. Usual set operations, but both operands must have the same relation schema. Selection: picking certain rows. Projection: picking certain columns. Products and joins: compositions of relations. Renaming of relations and attributes.

language for relational databases, is actually a bag language. Some operations, like projection, are more efficient on bags than sets. 31 Operations on Bags Selection applies to each tuple, so its effect on bags is like its effect on sets. Projection also applies to each tuple,

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