Yes!
That hint is just what I was looking for...

Declare
     @sSerie Varchar(10)
    ,@sFolio Varchar(10)

--Select @sSerie = '123'
Select @sFolio = '456'

Declare @xmlCfd xml
Set @xmlCfd = ''
Set @xmlCfd.modify('
    declare namespace cfd="http://www.sat.gob.mx/cfd/2";
    insert (
        <Comprobante>
        {
            if (sql:variable("@sSerie") != "")
            then attribute serie {sql:variable("@sSerie")}
            else ()
            ,if (sql:variable("@sFolio") != "")
            then attribute folio {sql:variable("@sFolio")}
            else ()
        }
        </Comprobante>
        )
        into (/)[1]'
    )

Select @xmlCfd

"Bob" <Bob@d...> escribió en el mensaje 
news:4E672E09-9EF5-46E7-96F1-A2337B311878@m......
> Got these two to work, but using element:
>
> DECLARE @sSerie VARCHAR(10),
> @sFolio VARCHAR(10)
>
> DECLARE @xmlCfd XML
>
> SET @xmlCfd = ''
> SET @sSerie = 2 -- Or 3
> SET @sFolio = 3
>
> -- Go easy on yourself and check the condition in SQL
> IF @sSerie = 2
> SET @xmlCfd.modify('declare namespace cfd="http://www.sat.gob.mx/cfd/2";
>   insert (<Comprobante>{sql:variable("@sSerie")}</Comprobante>)
>   into (/)[1]')
> ELSE
> SET @xmlCfd.modify('declare namespace cfd="http://www.sat.gob.mx/cfd/2";
>   insert (<Comprobante>{sql:variable("@sFolio")}</Comprobante>)
>   into (/)[1]')
>
> SELECT @xmlCfd
>
> -- Reset
> -- XQuery if method
> SET @xmlCfd = ''
> SET @xmlCfd.modify('declare namespace cfd="http://www.sat.gob.mx/cfd/2";
> insert
> if (sql:variable("@sSerie")="2")
> then element Comprobante {sql:variable("@sSerie")}
> else element Comprobante {sql:variable("@sFolio")}
> into (/)[1]')
>
> SELECT @xmlCfd
>
> This returns:
> <Comprobante>2</Comprobante>
>
> I know you mentioned attributes, but you can't have valid xml with
> attributes but no elements right?
>
> Post your expected results if this doesn't help.
>
> HTH
> wBob
>
> "SammyBar" wrote:
>
>> Hi,
>>
>> I'm experimenting with XML DML. I need to add an attribute only if it 
>> meet
>> some condition. But I can not use an sql variable to implement the
>> condition. The following example gives me an error:
>> XQuery [modify()]: An expression was expected
>>
>> Declare @sSerie Varchar(10)
>>   ,@sFolio Varchar(10)
>>
>> Declare @xmlCfd xml
>> Set @xmlCfd = ''
>> Set @xmlCfd.modify('
>>   declare namespace cfd="http://www.sat.gob.mx/cfd/2";
>>   insert (
>>     <Comprobante>
>>       {
>>         if ({sql:variable("@sSerie")} = 2)
>>         then attribute serie {sql:variable("@sSerie")}
>>         else ()
>>         ,attribute folio {sql:variable("@sFolio")}
>>       }
>>     </Comprobante>
>>   )
>>   into (/)[1]'
>> )
>>
>> Select @xmlCfd
>>
>> Obviously, the condition I need should look like
>> if ({sql:variable("@sSerie")} != '')
>>
>> How to implement such an expression?
>>
>> Any hint is welcomed
>> Thanks in advance
>> Sammy
>>
>>
>>