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The most typical relational algebra operators mapped to foundational SQL include unions and intersections, as well as other relational operators applied to these operators, including differences and various joins.
Basic set of operations for the relational model. Similar to algebra that operates on numbers. Operands and results are relations instead of numbers. Relational algebra expression. Composition of relational algebra operations. Possible because of closure property. Model for SQL.
Relational Algebra (1/3) The Basic Five operators: • Union: ∪ • Difference: - • Selection: σ • Projection: Π • Join: ⨝ Dan Suciu -- 444 Spring 2010
Binary Relational Operations: JOIN and DIVISION The JOIN Operation Denoted by Combine related tuples from two relations into single “longer” tuples General join condition of the form <condition> AND <condition> AND... AND <condition> Example:
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.
Relational algebra and relational calculus are formal languages for the relational model of data. A relation is a set of rows in a table with labeled columns. Relational algebra and associated operations are the basis for SQL. These relational operations define an algebra.
Useful when you need to perform a computation on values obtained by applying an aggregate. example: find the average enrollment in a CS course. SELECT AVG(count) FROM (SELECT course_name, COUNT(*) AS count FROM Enrolled GROUP BY course_name) WHERE course_name LIKE 'cs%';
