Oracle 11g Release 2

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CREATE TABLE T (COL_1, COL_2) AS
SELECT 'a1',      'b1' FROM DUAL UNION ALL
SELECT 'b1',      'a1' FROM DUAL UNION ALL
SELECT 'a2',      'b2' FROM DUAL UNION ALL
SELECT 'a3',      'b3' FROM DUAL UNION ALL
SELECT 'b3',      'a3' FROM DUAL
5 rows affected

SELECT * FROM T


COL_1
COL_2


a1
b1

b1
a1

a2
b2

a3
b3

b3
a3


SELECT ROWNUM, t1.COL_1, t1.COL_2
  FROM T t1
  INNER JOIN T t2
    ON t2.COL_2 = t1.COL_1 AND
       t2.COL_1 = t1.COL_2


ROWNUM
COL_1
COL_2


1
b1
a1

2
a1
b1

3
b3
a3

4
a3
b3


DELETE FROM T
  WHERE (COL_1, COL_2) IN (SELECT COL_1, COL_2
                             FROM (SELECT ROWNUM AS RN, t1.COL_1, t1.COL_2
                                     FROM T t1
                                     INNER JOIN T t2
                                       ON t2.COL_2 = t1.COL_1 AND
                                       t2.COL_1 = t1.COL_2)
                             WHERE RN / 2 <> TRUNC(RN / 2));
2 rows affected

SELECT * FROM T


COL_1
COL_2


a1
b1

a2
b2

a3
b3