SQL Server 2022

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    declare @TmpUno as table (
        uno int, dos varchar(max)
    )

    insert into @TmpUno (uno, dos)
    values (1, 'uno'), (2, 'dos')

declare @RC int, @XmlSerializedData xml;

set @XmlSerializedData =
(
    select uno, dos
    from @TmpUno
    for xml path('row'), type
);

set @RC = @XmlSerializedData.value('count(row)', 'int');

select @XmlSerializedData, @RC;


(No column name)
(No column name)


<row><uno>1</uno><dos>uno</dos></row><row><uno>2</uno><dos>dos</dos></row>
2


    declare @TmpUno as table (
        uno int, dos varchar(max)
    )

    insert into @TmpUno (uno, dos)
    values (1, 'uno'), (2, 'dos')
      
declare @RC int, @XmlSerializedData xml;

set @XmlSerializedData =
(
    select
      uno,
      dos,
      RowNumber = ROW_NUMBER() OVER (ORDER BY (SELECT 1))
    from @TmpUno
    for xml path('row'), type
);

set @RC = @XmlSerializedData.value('(row[last()]/RowNumber/text())[1]', 'int');

select @XmlSerializedData, @RC;


(No column name)
(No column name)


<row><uno>1</uno><dos>uno</dos><RowNumber>1</RowNumber></row><row><uno>2</uno><dos>dos</dos><RowNumber>2</RowNumber></row>
2


    declare @TmpUno as table (
        uno int, dos varchar(max)
    )

    insert into @TmpUno (uno, dos)
    values (1, 'uno'), (2, 'dos')
      
declare @RC int, @XmlSerializedData xml;

select @XmlSerializedData =
(
    select
      uno as [row/uno],
      dos as [row/dos],
      RowNumber =
             CASE WHEN
                  LEAD(0) OVER (ORDER BY (SELECT 1)) IS NULL
             THEN ROW_NUMBER() OVER (ORDER BY (SELECT 1)) END
    from @TmpUno
    for xml path(''), type
);

set @RC = @XmlSerializedData.value('(RowNumber/text())[1]', 'int');

select @XmlSerializedData, @RC;


(No column name)
(No column name)


<row><uno>1</uno><dos>uno</dos></row><row><uno>2</uno><dos>dos</dos></row><RowNumber>2</RowNumber>
2