Right, if you simply specify --
SELECT col1 FROM my_table WHERE rownum < 100;
what you will get are the first 100 records stored in the table, in no particular order. I've had developers try and "work around" it and do something like --
SELECT col1 FROM my_table WHERE rownum < 100 ORDER BY col1;
which looks like it will work, but doesn't because the rownum being a part of the WHERE clause gets evaluated before the ORDER BY clause.
Hence, the solution offered by beilstwh is the best solution... btw, Oracle calls that a "Top-N Query".