Eu usaria coalesce para isso:http://msdn.microsoft.com/en -us/library/ms190349.aspx
update Person
set Car = coalesce(@Car, Car), HairColour = coalesce(@HairColour, HairColour)
update Person
set Car = coalesce(@Car, Car), HairColour = coalesce(@HairColour, HairColour)
