PostgreSQL - WITH Clause

http://www.postgresql.org/docs/8.4/static/queries-with.html

http://www.tutorialspoint.com/postgresql/postgresql_with_clause.htm

http://www.craigkerstiens.com/2013/11/18/best-postgres-feature-youre-not-using/

http://beyondrelational.com/modules/2/blogs/80/posts/10748/day-6-common-table-expressions-using-with-clause-in-postgresql.aspx