Order by nested select

Question

I am using the following SELECT to fetch data from a database.

Is there a way to order the results by an item from the nested select? I tried ORDER BY B.ID but that creates an error.

My stored procedure:

SELECT      (
                SELECT      B.ID,
                            B.FirstName,
                            B.LastName
                FROM        Users B
                WHERE       B.UserNum = A.UserNum
                FOR XML     PATH(''), ELEMENTS, TYPE
            )
FROM        User_SolutionRole A
WHERE       A.Solution = 'SPR'
FOR XML PATH('users'), ELEMENTS, TYPE, ROOT('ranks')

Answer 1

If we could apply the values() method to the main SELECT's only column to extract the ID values from it, we could use the results to sort the rows, like this:

<column_ref>.values('ID[1]', 'int') ASC

It is actually possible to achieve that. If you assigned the column the name users and, at the same time, replaced PATH('users') with PATH(''), like this:

SELECT      (
                SELECT      B.ID,
                            B.FirstName,
                            B.LastName
                FROM        Users B
                WHERE       B.UserNum = A.UserNum
                FOR XML     PATH(''), ELEMENTS, TYPE
            ) AS users
FROM        User_SolutionRole A
WHERE       A.Solution = 'SPR'
FOR XML     PATH(''), ELEMENTS, TYPE, ROOT('ranks')
;

you would get the same output but the column would now have a reference. That would be your first step towards the goal.

It would seem you'd only need to add an ORDER BY like this:

SELECT      (
                SELECT      B.ID,
                            B.FirstName,
                            B.LastName
                FROM        Users B
                WHERE       B.UserNum = A.UserNum
                FOR XML     PATH(''), ELEMENTS, TYPE
            ) AS users
FROM        User_SolutionRole A
WHERE       A.Solution = 'SPR'
ORDER BY    users.value('ID[1]', 'int') ASC
FOR XML     PATH(''), ELEMENTS, TYPE, ROOT('ranks')
;

That wouldn't work, however, not at this point, because the alias assigned in a SELECT clause can only be referenced by the same level ORDER BY clause as is, i.e. it cannot be part of an expression. And so the above would result in a compilation error.

The solution to that would be to assign the name prior to the main SELECT. There are basically two ways:

1. Using a derived table:

   SELECT      users
   FROM        (
                   SELECT      (
                                   SELECT      B.ID,
                                               B.FirstName,
                                               B.LastName
                                   FROM        Users B
                                   WHERE       B.UserNum = A.UserNum
                                   FOR XML     PATH(''), ELEMENTS, TYPE
                               ) AS users
                   FROM        User_SolutionRole A
                   WHERE       A.Solution = 'SPR'
               ) AS s
   ORDER BY    users.value('ID[1]', 'int') ASC
   FOR XML     PATH(''), ELEMENTS, TYPE, ROOT('ranks')
   ;

   The above uses a "normal" subselect but it would be no difference if you declared it as a CTE instead.

2. Using CROSS APPLY:

   SELECT      x.users
   FROM        User_SolutionRole A
   CROSS APPLY (
                   SELECT      B.ID,
                               B.FirstName,
                               B.LastName
                   FROM        Users B
                   WHERE       B.UserNum = A.UserNum
                   FOR XML     PATH(''), ELEMENTS, TYPE
               ) AS x (users)
   WHERE       A.Solution = 'SPR'
   ORDER BY    x.users.value('ID[1]', 'int') ASC
   FOR XML     PATH(''), ELEMENTS, TYPE, ROOT('ranks')
   ;

My personal preference would be the CROSS APPLY technique as looking simpler and (to me) clearer, but the former method would work equally well.