Graph in RDBMS

        SELECT e3.*
        FROM link AS e3
        WHERE
            e3.id1 IN (
                SELECT e2.id2 
                FROM link AS e2
                WHERE
                    e2.id1 IN (
                        SELECT e1.id2
                        FROM link AS e1
                        WHERE
                            e1.id1 = 2006
                    )
            )

Good book (is it good? ;-) ): http://www.elsevier.com/wps/find/bookdescription.cws_home/702605/description#description

Articles by the same author: http://www.dbazine.com/ofinterest/oi-articles/celko24 and http://www.dbmsmag.com/9603d06.html

Discurrion: http://forum.vingrad.ru/index.php?showtopic=45952

Oracle Tip: Solving directed graph problems with SQL: http://builder.com.com/5100-6388_14-5245017.html

Trees in Postgres: http://archives.postgresql.org/pgsql-sql/1999-09/msg00092.php

Graphs for pg: http://pgfoundry.org/projects/pggraph/

http://www.cs.cmu.edu/~cache/pg_graph/