Common Table expression is a new feature introduced in Db2 version 8 used for recursive queying. This idea has been ported from databases like SQL server and Oracle which support recursive query be means of "CONNECT BY PRIOR". You can give a reading to the site below for how to use recursive querying in DB2.
www-128.ibm.com/developerworks/db2/library/techarticle/dm-0510rielau/
