It depends on the case, it can be a method, however 0 <> NULL, and <firstname xsi:nil="true"></firstname> is different from <fname></fname>.

 

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Cristian Andrei Lefter, SQL Server MVP
MCT, MCSA, MCDBA, MCAD, MCSD .NET,
MCTS, MCITP - Database Administrator SQL Server 2005
http://sqlserver.ro