with xR as (select 11 P_Id
            from dual),
    xPs as (select 11 P_Id,
                   444 Num,
                   333.22 SumTotal
            from dual),
     xP as (select 444 Num,
                   '123456789' Acc,
                   111.11 Sum
            from dual
            union all
            select 444 Num,
                   '987654321' Acc,
                   222.11 Sum
            from dual)

select xmlelement("Root",
         xmlforest(
           r.p_Id as "p_Id",
           xmlforest(
             ps.Num       as "Num",
             ps.SumTotal  as "SumTotal",
             xmlagg(
               xmlelement("Payment",
                 xmlforest(
                       p.Acc as "Acc",
                       p.Sum as "Sum"
                 )
               )
             ) "a"
           ) "Payments"
         )
       ) x
from xP p, xPs ps, xR r
where ps.P_Id = r.P_Id
and p.Num = ps.Num

<?xml version="1.0"?>
<Root>
  <p_Id>11</p_Id>
  <Payments>
    <Num>444</Num>
    <SumTotal>333.22</SumTotal>
    <a>
      <Payment>
        <Acc>123456789</Acc>
        <Sum>111.11</Sum>
      </Payment>
      <Payment>
        <Acc>987654321</Acc>
        <Sum>222.11</Sum>
      </Payment>
    </a>
  </Payments>
</Root>

<?xml version="1.0"?>
<Root>
  <p_Id>11</p_Id>
  <Payments>
    <Num>444</Num>
    <SumTotal>333.22</SumTotal>
    <Payment>
      <Acc>123456789</Acc>
      <Sum>111.11</Sum>
    </Payment>
    <Payment>
      <Acc>987654321</Acc>
      <Sum>222.11</Sum>
    </Payment>
  </Payments>
</Root>