Documentation Source Text

<p>Common Table Expressions or CTEs act like temporary [views] that exist
only for the duration of a single SQL statement.  There are two kinds of
common table expressions: "ordinary" and "recursive". Ordinary 
common table expressions are helpful for making
queries easier to understand by factoring
subqueries out of the main SQL statement.
Recursive common table expressions
provide the ability to do hierarchical or
recursive queries of trees and graphs, a capability
that is not otherwise available in the SQL language.

<p>All common table expressions (ordinary and recursive) are 
created by prepending a WITH clause in front of a [SELECT], [INSERT], [DELETE],
or [UPDATE] statement.  A single WITH clause can specify one or more
common table expressions, some of which are ordinary and some of which
are recursive.

<tcl>hd_fragment ordinarycte {ordinary common table expressions}</tcl>
<h3>Ordinary Common Table Expressions</h3>

<p>An ordinary common table expression works as if it were a [view] that
exists for the duration of a single statement.  Ordinary common table
expressions are useful for factoring out subqueries and making the overall
SQL statement easier to read and understand.

<p>A WITH clause can contain ordinary common table expressions even if
it includes the RECURSIVE keyword.  The use of RECURSIVE does not force
common table expressions to be recursive.

<tcl>
hd_fragment recursivecte {recursive common table expressions} \
{recursive query}
</tcl>
<h3>Recursive Common Table Expressions</h3>