SQL> select * from visit;

 PATIENTID   POLICEID
---------- ----------
        10        100

SQL> select * from polis;

        ID  PATIENTID
---------- ----------
         1         10
         2         10
         1         20
         3         30

SQL> ed
Wrote file afiedt.buf

  1  merge
  2       into visit v
  3       using(
  4           select  max(id) mid, patientid
  5           from  polis
  6           group by patientid
  7            ) p
  8      on
  9        (v.patientid = p.patientid)
 10    when matched
 11    then update set policeid = p.mid
 12*   when not matched then insert values (p.patientid, p.mid)
SQL> /

3 rows merged.

SQL> select * from visit;

 PATIENTID   POLICEID
---------- ----------
        10          2
        30          3
        20          1

SQL>