Chapter08ARelationalAlgebra (1).pptx

# Unary relational operations select select operation

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Unary Relational Operations: SELECT SELECT Operation Properties The SELECT operation <selection condition> (R) produces a relation S that has the same schema (same attributes) as R SELECT is commutative: <condition1> ( < condition2> (R)) = <condition2> ( < condition1> (R)) Because of commutativity property, a cascade (sequence) of SELECT operations may be applied in any order: <cond1> ( <cond2> ( <cond3> (R)) = <cond2> ( <cond3> ( <cond1> ( R))) A cascade of SELECT operations may be replaced by a single selection with a conjunction of all the conditions: <cond1> ( < cond2> ( <cond3> (R)) = <cond1> AND < cond2> AND < cond3> (R))) The number of tuples in the result of a SELECT is less than (or equal to) the number of tuples in the input relation R Relational Algebra and Calculus 9

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Query Results Refer to this Database State Relational Algebra and Calculus 10
Unary Relational Operations: PROJECT PROJECT Operation is denoted by  (pi) This operation keeps certain columns (attributes) from a relation and discards the other columns. PROJECT creates a vertical partitioning The list of specified columns (attributes) is kept in each tuple The other attributes in each tuple are discarded Example: To list each employee’s first and last name and salary, the following is used: LNAME, FNAME,SALARY (EMPLOYEE) Relational Algebra and Calculus 11

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Unary Relational Operations: PROJECT The general form of the project operation is: <attribute list> (R) (pi) is the symbol used to represent the project operation <attribute list> is the desired list of attributes from relation R. The project operation removes any duplicate tuples This is because the result of the project operation must be a set of tuples Mathematical sets do not allow duplicate elements. Relational Algebra and Calculus 12
Unary Relational Operations: PROJECT PROJECT Operation Properties The number of tuples in the result of projection <list> (R) is always less or equal to the number of tuples in R If the list of attributes includes a key of R, then the number of tuples in the result of PROJECT is equal to the number of tuples in R PROJECT is not commutative <list1> ( <list2> (R) ) = <list1> (R) as long as <list2> contains the attributes in <list1> Relational Algebra and Calculus 13

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Examples of Applying SELECT and PROJECT Operations Relational Algebra and Calculus 14
Relational Algebra Expressions We may want to apply several relational algebra operations one after the other Either we can write the operations as a single relational algebra expression by nesting the operations, or We can apply one operation at a time and create intermediate result relations .

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• Fall '09
• SUNANHAN

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