Return row of every n'th record

How can I return every nth record from a sub query based on a numerical parameter that I supply?

For example, I may have the following query:

SELECT
   Id,
   Key
FROM DataTable
WHERE CustomerId = 1234
ORDER BY Key

e.g.

The subquery result may look like the following:

Row Id   Key
1   1    A3231
2   43   C1212
3   243  E1232
4   765  G1232
5   2432 E2325
...
90  3193 F2312

If I pass in the number 30, and the sub query result set contained 90 records, I would recieve the 30th, 60th, and 90th row.

If I pass in the number 40, and the result set contained 90 records, I would recieve the 40th and 80th row.

As a side note, for background information, this is being used to capture the key/id of every nth record for a paging control.

This is where ROW_NUMBER can help. It requires an order-by clause but this is okay because an order-by is present (and required to guarantee a particular order).

SELECT t.id, t.key
FROM
(
    SELECT id, key, ROW_NUMBER() OVER (ORDER BY key) AS rownum
    FROM datatable
) AS t
WHERE t.rownum % 30 = 0    -- or % 40 etc
ORDER BY t.key

You don't have to use row number, any integer field will do. On most tables we have an autonumber field. For simplicity, I will call it ID. To display every 13th record:

Select ID, LastName, FirstName FROM Users WHERE ID%13 = 0 ORDER BY ID

Select B.name 
from (Select row_number() over (order by ordering) as row_num, 
      A.name 
      from (Select name,'P1' as ordering 
            from [dbo].[every_2nd_record]
           ) A
      ) B
where B.row_num%2=0

Return row of every n'th record

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