Thanks Jeff and prabodh.
So far as "vtables" go, are you talking about "Table Variables"?
No.
I understood the difference between =NULL,ISNULL and SET ANSIL NULL ON/OFF after reading the below article.
http://www.sqlservercentral.com/articles/Basic+Querying/understandingthedifferencebetweenisnull/871/
In this article, he has used vtable, just i wanted to know more about that.
karthik