create table t1 as select level id, decode(level, 1, trunc(sysdate)) dt from dual connect by level <= 5;

Таблица создана.


select * from t1;

        ID DT
---------- -------------------
         1 14.03.2017 00:00:00
         2
         3
         4
         5


merge into t1 tgt
using
    (select id, first_value(sysdate) over (order by id) + row_number () over (order by id) - 1 dt
     from t1
    ) src on (tgt.id = src.id)
when matched then update set
    tgt.dt = src.dt
where decode(tgt.dt, src.dt, 1) is null;

5 строк объединено.


select * from t1;

        ID DT
---------- -------------------
         1 14.03.2017 18:01:23
         2 15.03.2017 18:01:23
         3 16.03.2017 18:01:23
         4 17.03.2017 18:01:23
         5 18.03.2017 18:01:23


drop table t1 purge;

Таблица удалена.

    (select id, first_value(sysdate) over (order by id) + row_number () over (order by id) - 1 dt

 (select id, first_value(/*sysdate*/dt) over (order by id) + (row_number () over (order by id) - 1) * interval '15' minute dt