+-------+---------+
| empid | empname |
+-------+---------+
|     1 | Arul    |
|     2 | Ram     |
|     3 | Nivi    |
+-------+---------+

+-------+------------+----------------+
| empid | empaddress |    empemail    |
+-------+------------+----------------+
|     1 | Chennai    | mail1@mail.com |
|     2 | Madurai    | mail2@mail.com |
|     3 | Mumbai     | NULL           |
+-------+------------+----------------+

select A.empid,A.empname,
(select B.empaddress,isnull(B.empemail,'') as empemail from empdetail B where A.empid=B.empid  for xml path('EmployeeDetails'),type)
from emp A
where A.empid=1 for xml path('root')

<root>
  <empid>1</empid>
  <empname>Arul</empname>
  <EmployeeDetails>
    <empaddress>Chennai</empaddress>
    <empemail>mail1@mail.com</empemail>
  </EmployeeDetails>
</root>

<root>
  <empid>3</empid>
  <empname>Nivi</empname>
  <EmployeeDetails>
    <empaddress>Mumbai</empaddress>
    <empemail />
  </EmployeeDetails>
</root>

<root>
  <empid>3</empid>
  <empname>Nivi</empname>
  <EmployeeDetails>
    <empaddress>Mumbai</empaddress>
    <empemail></empemail>
  </EmployeeDetails>
</root>