I don't think a natural join specifically exists in TSQL... And the only thing I can think of what it could mean is a normal join between 2 tables with the same ID in the ON, something like:
FROM Orders O
INNER JOIN Clients C ON C.ClientID = O.ClientID
with what makes it natural that ClientID exists in both tables.
Ronald HensbergenHelp us, help yourself... Post data so we can read and use it: http://www.sqlservercentral.com/articles/Best+Practices/61537/-------------------------------------------------------------------------2+2=5 for significant large values of 2