update person set height = '171m' where rownum < 501;
update person set height = '175m' where height <> '171m' and rownum < 501;
update person set height = '173m' where height <> '171m' and height <> '175m';
update person set height = '175m' where height <> '171m' and rownum < 501;
update person set height = '173m' where height <> '171m' and height <> '175m';
update person set height='175m' where height is null and rownum <= 500;
update person set height='173m' where height is null and rownum <= 500;
update person set height = '175m' where height <> '171m' and rownum < 501;