SQL Server 2005 - Row_Number() - sql

Question

I'm trying to understand the unusual behaviour seen when ordering results in a descending order using the row_number() function when using a DISITINCT on the outermost select in my query as below:
SELECT DISTINCT (ID), State_Id, Name_Of_Trip, Date_Of_Travel, Creation_Date, Locking_Id, Applicant_Name, Reference_Number, State_Name
FROM (
SELECT app.ID, app.State_Id, app.Name_Of_Trip, app.Date_Of_Travel, app.Creation_Date, app.Locking_Id, app.Applicant_Name, app.Reference_Number,
State.Name AS State_Name, ROW_NUMBER() OVER(ORDER BY Reference_Number DESC) as rowNum
FROM Application_Leg AS app
INNER JOIN State AS state
ON app.State_Id = state.ID
WHERE (app.State_Id = 5 OR app.State_Id = 6 OR app.State_Id = 8) AND app.Organisation_Id=12
AND Leg_Number IN
(SELECT DISTINCT Leg_Number
from Application_Leg as al
INNER JOIN
Organisation as org
ON al.Organisation_Id = org.ID
WHERE al.ID=app.ID AND org.Approval_Required=1 AND Mode_Of_Transport=1))
as pagedApplications
WHERE rowNum BETWEEN 0 AND (0 + 10)
When the outermost DISTINCT is taken out then the descending order is fine but when it is included the results are not shown in descending order.

Answer 1

ORDER BY in ROW_NUMBER clause does not guarantee the order of the resultset.
ROW_NUMBER usually uses sorting in the query plan which results in the fact that the values come out presorted.
This is a side effect and should not be relied upon.
DISTINCT uses Hash Match (Aggregate) which breaks sorting.
Add ORDER BY clause to the end of the query:
SELECT DISTINCT (ID), State_Id, Name_Of_Trip, Date_Of_Travel, Creation_Date, Locking_Id, Applicant_Name, Reference_Number, State_Name
FROM (
SELECT app.ID, app.State_Id, app.Name_Of_Trip, app.Date_Of_Travel,
app.Creation_Date, app.Locking_Id, app.Applicant_Name, app.Reference_Number,
State.Name AS State_Name, ROW_NUMBER() OVER(ORDER BY Reference_Number DESC) as rowNum
FROM Application_Leg AS app
INNER JOIN
State AS state
ON app.State_Id = state.ID
WHERE app.State_Id IN (5, 6, 8)
AND app.Organisation_Id = 12
AND Leg_Number IN
(
SELECT Leg_Number
FROM Application_Leg as al
INNER JOIN
Organisation as org
ON al.Organisation_Id = org.ID
WHERE al.ID = app.ID
AND org.Approval_Required = 1
AND Mode_Of_Transport = 1
)
) AS pagedApplications
WHERE rowNum BETWEEN 0 AND (0 + 10)
ORDER BY
ReferenceNumber DESC
Also note that it will not return 10 distinct results, it will return DISTINCT of the first 10 results.
If you want the former, use this:
SELECT DISTINCT TOP 10 ID, State_Id, Name_Of_Trip, Date_Of_Travel, Creation_Date, Locking_Id, Applicant_Name, Reference_Number, State_Name
FROM (
SELECT app.ID, app.State_Id, app.Name_Of_Trip, app.Date_Of_Travel,
app.Creation_Date, app.Locking_Id, app.Applicant_Name, app.Reference_Number,
State.Name AS State_Name
FROM Application_Leg AS app
INNER JOIN
State AS state
ON app.State_Id = state.ID
WHERE app.State_Id IN (5, 6, 8)
AND app.Organisation_Id = 12
AND EXISTS
(
SELECT Leg_Number
FROM Application_Leg AS al
INNER JOIN
Organisation as org
ON al.Organisation_Id = org.ID
WHERE al.ID = app.ID
AND al.LegNumber = app.LegNumber
AND org.Approval_Required = 1
AND Mode_Of_Transport = 1
)
) AS pagedApplications
ORDER BY
ReferenceNumber DESC

Answer 2

Have you tried adding an order by to your outer select?