General trees persisted in relational databases - The Code Project - C# Database
http://www.codeproject.com/cs/database/persisting_trees.asp
Sql
PostgreSQL: PostgreSQL 7.3 Set Returning Functions
http://www.postgresql.org/docs/techdocs.17 returning multiple rows from a function in postgresql
Library of Free Data Models from DatabaseAnswers.org
http://www.databaseanswers.org/data_models/index.htm useful data model samples
Trees in SQL databases - The Code Project - C# Database
http://www.codeproject.com/cs/database/Trees_in_SQL_databases.asp how to create tree in sql database
Comparison of different SQL implementations
http://troels.arvin.dk/db/rdbms/