Significance Of Null Value

Check this link for what NULL values are and how they can be handled:

http://www.techrepublic.com/blog/datacenter/handling-null-values-in-sql-server-2005/153 

I don't think it's a good idea to avoid using NULL values altogether, just because they're difficult to deal with. There are many occasions in database work where data will be missing and there's no sensible default value which you can use instead.