RELATIONAL ALGEBRA

RELATIONAL ALGEBRA

-A query language (QL) is a language that allows users to manipulate and retrieve data from a database.

-Relational Algebra is useful for representing query execution plans, and query optimization techniques.

-Relational Algebra comprises a set of basic operations.

-An operation is the application of an operator to one or more source (or input) relations to produce a new relation as a result.

-Relational algebra has the property of closure, that is the output from each of the operations is another relation.

5 BASIC RA OPERATIONS

•Projection ( ) Retains only wanted columns from relation (vertical).

•Selection ( ) Selects a subset of rows from relation (horizontal).

•Cross-product (x) Allows us to combine two relations.

•Set-difference (–) Tuples in R1, but not in R2.

•Union (È ) Tuples in R1 and/or in R2.

NOTE: R1 = relation/table 1

R2 = relation/table 2