Question

DECLARE @OrdersTemp TABLE
(
   OrderId UNIQUEIDENTIFIER
)

INSERT INTO @OrdersTemp
  SELECT ord.Id
  FROM Orders 

--all rows count
SELECT  
    @RowsCount = COUNT(DISTINCT ord.Id)
FROM   Orders 


--@RowsCount = 5. It's right!

--second table with paging
DECLARE @OrdersTempWithPaging TABLE
(
    OrderId UNIQUEIDENTIFIER
)

INSERT INTO @OrdersTempWithPaging
   SELECT OrderId
   FROM  (SELECT DISTINCT OrderId,
                 ROW_NUMBER() OVER (ORDER BY OrderId) AS RowNum
          FROM @OrdersTemp) AS alias
          WHERE 
              RowNum BETWEEN (@PageIndex - 1) * @PageSize + 1
                     AND @PageIndex * @PageSize

SELECT * FROM @OrdersTempWithPaging
--10 or more rows. It's wrong.

Why does @OrdersTempWithPaging return wrong amount of rows? How do I avoid it?

UPDATE:
The statement below returns 25 = 5*5 rows (instead of 5)

   INSERT  INTO @OrdersTempWithPaging
                SELECT    OrderId
                FROM    (
                          SELECT    OrderId,
                                    ROW_NUMBER() OVER (ORDER BY OrderId ) AS RowNum
                          FROM      @OrdersTemp ) AS alias
                --WHERE   RowNum BETWEEN ( @PageIndex - 1 ) * @PageSize + 1
                             --  AND     @PageIndex * @PageSize
  SELECT * FROM @OrdersTempWithPaging

Answer 1

Try this (reversing the DISTINCT use):

INSERT INTO @OrdersTempWithPaging
   SELECT DISTINCT OrderId
   FROM  (SELECT OrderId,
                 ROW_NUMBER() OVER (ORDER BY OrderId) AS RowNum
          FROM @OrdersTemp) AS alias
   WHERE 
       RowNum BETWEEN (@PageIndex - 1) * @PageSize + 1
              AND @PageIndex * @PageSize

If you need only distinct order-ids, you could have:

INSERT INTO @OrdersTemp
  SELECT DISTINCT ord.Id
  FROM Orders 

and then:

INSERT INTO @OrdersTempWithPaging
   SELECT OrderId
   FROM  (SELECT OrderId,
                 ROW_NUMBER() OVER (ORDER BY OrderId) AS RowNum
          FROM @OrdersTemp) AS alias
   WHERE 
       RowNum BETWEEN (@PageIndex - 1) * @PageSize + 1
              AND @PageIndex * @PageSize