>>By default all declared variables will be NULL. Assuming you will not be having NULL values in your XML file you can do
>>
>>update myTable SET Fld1Name = COALESCE(@Fld1Value, Fld1Name), etc.
>
>I didn't know that I can skip declaring a variable. I thought that I always have to do
>declare @MyVarName INT before using it.
>
>I have never used COALESCE() either. The approach you describe should work. I will test it.
>Thank you.

This is not what I said. You can not skip DECLARE in SQL Server, all variables must be declared. Once a variable is declared, it will be NULL. The select @FidlVar = ...
doesn't change the value if that select statement returns 0 rows. So, if you never going to have NULL values passed in your XML, then the solution I suggested is the simplest one.

The alternative is to build the dynamic SQL which is also simple.