Summary

This document covers relational algebra, a language for defining and manipulating data in relational databases. The document includes examples of how to use relational algebra operations such as selecting and projecting tuples.

Full Transcript

Filters the attributes/columns of a resulting relation R based on the attributes RELATIONAL ALGEBRA listed...

Filters the attributes/columns of a resulting relation R based on the attributes RELATIONAL ALGEBRA listed “select” clause Not (entirely) commutative Procedural language Relations are the inputs - not correct to begin with Resulting relations are the outputs Represent a query in a “mathematical” way - not commutative but proper use of projection Example: OPERATIONS IN RELATIONAL ALGEBRA 1. Show all first and last names of employees. πfirstname, lastname(employee) ○ resulting relation has more or less number of attributes Select operation from the original relation and with the original number of σcondition(relation) tuples where: 2. Show all last names, department numbers, monthly salaries of employees - σ: select symbol πlastname, department, salary(employee) - condition: condition based on attribute(s) relation is the relation to be used Note: You can combine selection and projection (which normally happens) Filters the tuples of a resulting relation R based on the condition provided “where” clause commutative Example: Show the last names and department numbers of employees whose monthly salary is greater than Php 30,000. Rename operation Example: 1. Show all employees who belongs to department number 5. ⍴new_relation(new_attributes)(old_relation) σdepartment=5(employee) where: ○ resulting relation has the same number of attributes from ⍴: select symbol the original relation but with more or less tuples (rows) new_relation(new_attributes): new relation/ attributes 2. Show all employees whose monthly salary is above Php30,000 and belongs old_relation: relation to be used to department 3, 4, and 5. σsalary > 30000 and department >=3 and department

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