Oracle Update fra Select - samme tabel

Du bør kunne:

update t
    set date1 = greatest(date1, coalesce(date2, date1), coalesce(date3, date1), coalesce(date4, date1))
    where date1 < greatest(date1, coalesce(date2, date1), coalesce(date3, date1), coalesce(date4, date1));

I dine data, date1 ser ud til aldrig at være NULL .