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Chapter 43: Relational Algebra

Examples

Overview

Relational Algebra is not a full-blown SQL language, but rather a way to gain theoretical
understanding of relational processing. As such it shouldn't make references to physical entities
such as tables, records and fields; it should make references to abstract constructs such as
relations, tuples and attributes. Saying that, I won't use the academic terms in this document and
will stick to the more widely known layman terms - tables, records and fields.

A couple of rules of relational algebra before we get started:

• The operators used in relational algebra work on whole tables rather than individual records.
• The result of a relational expression will always be a table (this is called the closure property)

Throughout this document I will be referring to the follow two tables:

SELECT

The select operator returns a subset of the main table.
select < table > where < condition >

For example, examine the expression:

select People where DepartmentID = 2

This can be written as:

This will result in table whose records comprises of all records in the People table where the
DepartmentID value is equal to 2:

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