Is order by clause allowed in a subquery

Question

Is there any reason why or why not you should do an \'order by\' in a subquery?

Answer 1

You should use it if the subquery uses some kind of LIMIT / TOP.

SQL Server will not allow it unless the subquery contains TOP or FOR XML clause as well:

-- Fails
WITH    q(id) AS
        (
        SELECT  1
        UNION ALL
        SELECT  2
        )
SELECT  *
FROM    (
        SELECT  *
        FROM    q
        ORDER BY
                id DESC
        ) q2

-- Succeeds
WITH    q(id) AS
        (
        SELECT  1
        UNION ALL
        SELECT  2
        )
SELECT  *
FROM    (
        SELECT  TOP 1 *
        FROM    q
        ORDER BY
                id DESC
        ) q2

-- Succeeds, but ORDER BY is ignored
WITH    q(id) AS
        (
        SELECT  1
        UNION ALL
        SELECT  2
        )
SELECT  *
FROM    (
        SELECT  TOP 100 PERCENT *
        FROM    q
        ORDER BY
                id DESC
        ) q2