match_recognize
を使用したエレガントな(そして効率的な)ソリューションがあります 句(Oracle 12.1以降が必要です)。
select po, startdate, enddate
from orders
match_recognize (
partition by po
order by startdate
measures first(startdate) as startdate, max(enddate) as enddate
pattern ( c* n )
define c as max(enddate) + 1 >= next(startdate)
);