And the use of rownum, which is a pseudo column, won't always return the same rows. It depends on what else you are doing (sort, order, group by, having, and doing nothing to sort the data may return entirely different rows). If you don't care which 10 rows come out, then what Ao wrote will work for you.

Also, you can only do "where rownum=1" or "where rownum < some_number." For example, you can't do "where rownum=5."