Database

http://www.codeproject.com/cs/database/persisting_trees.asp

http://blogs.sun.com/jkshah/

http://www.postgresql.org/docs/techdocs.2 collection of contributed articles on PostgresSQL

http://www.databaseanswers.org/data_models/index.htm useful data model samples

http://www.codeproject.com/cs/database/Trees_in_SQL_databases.asp how to create tree in sql database