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 
> 
> 
>