SQL> select a from t_num;

                                      A
---------------------------------------
                                      1
                                      2
                                      3
                                     30
                                   -230

SQL> select a,first_value(a) over(order by a rows unbounded preceding) from t_num;

                                      A FIRST_VALUE(A)OVER(ORDERBYAROW
--------------------------------------- ------------------------------
                                   -230                           -230
                                      1                           -230
                                      2                           -230
                                      3                           -230
                                     30                           -230

--注：提取a列的最小值;last_value同理