Return row of every n’th record

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