CREATE TABLE distance
  (
     city1 VARCHAR2(20),
     city2 VARCHAR2(20),
     dist  NUMBER
  );

INSERT INTO distance
VALUES      ('DELHI',
             'MUMBAI',
             4000);

INSERT INTO distance
VALUES      ('MUMBAI',
             'DELHI',
             4000);

INSERT INTO distance
VALUES      ('DELHI',
             'PUNE',
             2000);

INSERT INTO distance
VALUES      ('PUNE',
             'DELHI',
             2000);

INSERT INTO distance
VALUES      ('MUMBAI',
             'PUNE',
             6000);

INSERT INTO distance
VALUES      ('PUNE',
             'MUMBAI',
             6000);

COMMIT; 

select * from distance d1
where not exists(Select 1 from distance d2
                  where d1.CITY1||d1.CITY2 = d2.CITY2||d2.CITY1 );

INSERT INTO distance VALUES ('PARIS','MARSEILLE',800);

select  least(city1,city2) city1,
        greatest(city1,city2) city2,
        dist
  from  distance
  group by least(city1,city2),
           greatest(city1,city2),
           dist
/

CITY1                CITY2                      DIST
-------------------- -------------------- ----------
MUMBAI               PUNE                       6000
DELHI                MUMBAI                     4000
DELHI                PUNE                       2000

SQL> 

SQL> create unique index distance_uidx1
  2    on distance(
  3                least(city1,city2),
  4                greatest(city1,city2)
  5               )
  6  /

Index created.

SQL> INSERT INTO distance
  2  VALUES      ('DELHI',
  3               'MUMBAI',
  4               4000);

1 row created.

SQL> 
SQL> INSERT INTO distance
  2  VALUES      ('MUMBAI',
  3               'DELHI',
  4               4000);
INSERT INTO distance
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.DISTANCE_UIDX1) violated


SQL> 

WITH match 
     AS (SELECT d1.city1, 
                Max(d1.rowid) mrid 
         FROM   distance d1 
         GROUP  BY d1.city1) 
SELECT d1.city1, 
       d1.city2, 
       d1.dist 
FROM   match, 
       distance d1 
WHERE  d1.rowid = match.mrid 

select *
from
(
        select  city1,
                city2,
                dist,
                row_number() over (partition by dist order by dist) as rnk
        from    distance
 )
 where rnk = 1;

SQL> select * from distance
  2  /

CITY1                CITY2                      DIST
-------------------- -------------------- ----------
DELHI                MUMBAI                     4000
MUMBAI               DELHI                      4000
DELHI                PUNE                       2000
PUNE                 DELHI                      2000
MUMBAI               PUNE                       6000
PUNE                 MUMBAI                     6000

6 rows selected.

SQL> delete from distance
  2  where rowid not in
  3  (
  4   select min(rowid)
  5   from distance
  6   group by dist
  7  )
  8  /

3 rows deleted.

SQL> select * from distance
  2  /

CITY1                CITY2                      DIST
-------------------- -------------------- ----------
DELHI                MUMBAI                     4000
DELHI                PUNE                       2000
MUMBAI               PUNE                       6000

select *
from
(
        select  city1,
                city2,
                dist,
                row_number() over (partition by dist order by city1,city2,dist) as rnk
        from    distance
 )
 where rnk = 1;

INSERT INTO distance VALUES ('PARIS','ATHENS',2000);

CITY1	CITY2	DIST	RNK

PARIS	ATHENS	2000	1
DELHI	MUMBAI	4000	1
MUMBAI	PUNE	6000	1

select city2, city1 ,dist from(
select rownum id, a.* from distance a) 
where id in (select id+(id+1)-1 from (select rownum id, a.* from distance a))

id+(id+1)-1

id*2

select city2, city1 ,dist from(
select rownum id, a.* from distance a) 
where mod(id,2) = 0;

SQL> select * from distance;
CITY1                CITY2                      DIST
-------------------- -------------------- ----------
DELHI                MUMBAI                     4000
MUMBAI               DELHI                      4000
DELHI                PUNE                       2000
PUNE                 DELHI                      2000
MUMBAI               PUNE                       6000
PUNE                 MUMBAI                     6000
PARIS                MARSEILLE                   800
PARIS                ATHENS                     2000

8 rows selected.

SQL> select  least(city1,city2) city1,
  2          greatest(city1,city2) city2,
  3          dist
  4    from  distance
  5    group by least(city1,city2),
  6             greatest(city1,city2),
  7             dist
  8  /
CITY1                CITY2                      DIST
-------------------- -------------------- ----------
MUMBAI               PUNE                       6000
MARSEILLE            PARIS                       800
ATHENS               PARIS                      2000
DELHI                MUMBAI                     4000
DELHI                PUNE                       2000

5 rows selected.