RELATIONAL ALGEBRA

Relational Algebra

• Formal language for the relational model

• defines the basic set of operations for the relational model for basic retrieval requests

SELECT, PROJECT, JOIN

Operations specifically developed for relational databases

UNION, INTERSECTION, SET DIFFERENCE, CARTESIAN PRODUCT

Mathematical Operations on Sets

Unary Relational Operations

applied to a single relation

SELECT

• select a subset of the tuples from a relation that satisfies a selection condition

• a boolean expression specified on the attributes of relation R

PROJECT

selects certain columns from the table and discards the other columns; used if we are interested only in certain attributes of a relation

UNION

• relation that includes all tuples that are either in R or in S or in both R and S

• Duplicate tuples are eliminated

INTERSECTION

relation that includes all tuples that are in both R and S

MINUS (SET DIFFERENCE)

relation that includes all tuples that are in R but not in S

Union compatibility

Relations should have same degree (same number of attributes) and each corresponding pair of attributes have same domain

CARTESIAN PRODUCT

• Combines every tuple from one relation with every tuple from another relation

• not union compatible

CARTESIAN PRODUCT

• will have a degree of n+m attributes

• will have one tuple for each combination of tuples, one from R and one from S

• will have nR * nS tuples

Binary Relational Operations

applied to two sets of tuples

JOIN

used to combine related tuples from 2 relations into a single tuple

JOIN

has one tuple for each combination of tuples --- one from R and one from S --- whenever the combination satisfies the join condition.