Help with remove reversed duplicate rows?

Question

Hi:
I have an application where I need to remove reversed duplicate rows, such as this:
select 'A' x, 'B' y from dual
union all
select 'C', 'D' from dual
union all
select 'B','A' from dual;
Output is:
X Y
A B
C D
B A
What I need is to remove the 'B A' row because it already exists in reverse form as 'A B'. It does not matter which row is kept, 'A B' or 'B A'. Just one of them needs to be removed and the other needs to stay.
Note that the actual query to generate the data is a little more involved, but I can use a with clause to make it an inline view. I'm hoping that the actual mechanics are not relevant. If that is not the case, please let me know and I'll post more details about the actual query.
Any ideas?

Answer 1

  WITH T AS
( SELECT  'HOUSE' x, 'HOUSE' y FROM DUAL UNION ALL
SELECT 'HOUSE', 'JME0609U' FROM DUAL UNION ALL
SELECT 'HOUSE', 'JME4Q08' FROM DUAL UNION ALL
SELECT 'HOUSE', 'JMEM' FROM DUAL UNION ALL
SELECT 'HOUSE', 'MATCH2' FROM DUAL UNION ALL
SELECT 'HOUSE', 'MATCH3' FROM DUAL UNION ALL
SELECT 'HOUSE', 'PGCM' FROM DUAL UNION ALL
SELECT 'HOUSE', 'PGCU' FROM DUAL UNION ALL
SELECT 'HOUSE', 'VEL' FROM DUAL UNION ALL
SELECT 'HOUSE', 'XXX08' FROM DUAL UNION ALL
SELECT 'JME0609U', 'HOUSE' FROM DUAL UNION ALL
SELECT 'JME0609U', 'JME0609U' FROM DUAL UNION ALL
SELECT 'JME0609U', 'MATCH3' FROM DUAL UNION ALL
SELECT 'JME0609U', 'PGCM' FROM DUAL UNION ALL
SELECT 'JME0609U', 'PGCU' FROM DUAL UNION ALL
SELECT 'JME0609U', 'VEL' FROM DUAL UNION ALL
SELECT 'JME0609U', 'XXX08' FROM DUAL UNION ALL
SELECT 'JMEM', 'HOUSE' FROM DUAL UNION ALL
SELECT 'JMEM', 'JMEM' FROM DUAL UNION ALL
SELECT 'JMEM', 'MATCH3' FROM DUAL UNION ALL
SELECT 'JMEM', 'PGCM' FROM DUAL UNION ALL
SELECT 'JMEM', 'PGCU' FROM DUAL UNION ALL
SELECT 'JMEM', 'VEL' FROM DUAL UNION ALL
SELECT 'JMEM', 'XXX08' FROM DUAL UNION ALL
SELECT 'MATCH2', 'HOUSE' FROM DUAL UNION ALL
SELECT 'MATCH2', 'MATCH2' FROM DUAL UNION ALL
SELECT 'MATCH2', 'MATCH3' FROM DUAL UNION ALL
SELECT 'MATCH2', 'PGCM' FROM DUAL UNION ALL
SELECT 'MATCH2', 'PGCU' FROM DUAL UNION ALL
SELECT 'MATCH2', 'VEL' FROM DUAL UNION ALL
SELECT 'MATCH2', 'XXX08' FROM DUAL UNION ALL
SELECT 'MATCH3', 'HOUSE' FROM DUAL UNION ALL
SELECT 'MATCH3', 'JME0609U' FROM DUAL UNION ALL
SELECT 'MATCH3', 'JMEM' FROM DUAL UNION ALL
SELECT 'MATCH3', 'MATCH2' FROM DUAL UNION ALL
SELECT 'MATCH3', 'MATCH3' FROM DUAL UNION ALL
SELECT 'MATCH3', 'XXX08' FROM DUAL UNION ALL
SELECT 'PGCM', 'HOUSE' FROM DUAL UNION ALL
SELECT 'PGCM', 'JME0609U' FROM DUAL UNION ALL
SELECT 'PGCM', 'JMEM' FROM DUAL UNION ALL
SELECT 'PGCM', 'MATCH2' FROM DUAL UNION ALL
SELECT 'PGCM', 'PGCM' FROM DUAL UNION ALL
SELECT 'PGCM', 'XXX08' FROM DUAL UNION ALL
SELECT 'PGCU', 'HOUSE' FROM DUAL UNION ALL
SELECT 'PGCU', 'JME0609U' FROM DUAL UNION ALL
SELECT 'PGCU', 'JMEM' FROM DUAL UNION ALL
SELECT 'PGCU', 'MATCH2' FROM DUAL UNION ALL
SELECT 'PGCU', 'PGCU' FROM DUAL UNION ALL
SELECT 'PGCU', 'XXX08' FROM DUAL UNION ALL
SELECT 'VEL', 'HOUSE' FROM DUAL UNION ALL
SELECT 'VEL', 'JME0609U' FROM DUAL UNION ALL
SELECT 'VEL', 'JMEM' FROM DUAL UNION ALL
SELECT 'VEL', 'MATCH2' FROM DUAL UNION ALL
SELECT 'VEL', 'VEL' FROM DUAL UNION ALL
SELECT 'VEL', 'XXX08' FROM DUAL )
SELECT x, y
FROM   (SELECT x, y, row_number() over (partition by least(x,y), greatest(x,y) order by rownum) rn
        FROM   t)
WHERE  rn = 1
HOUSE,HOUSE
HOUSE,JME0609U
HOUSE,JME4Q08
HOUSE,JMEM
HOUSE,MATCH2
HOUSE,MATCH3
HOUSE,PGCM
HOUSE,PGCU
HOUSE,VEL
HOUSE,XXX08
JME0609U,JME0609U
JME0609U,MATCH3
JME0609U,PGCM
JME0609U,PGCU
JME0609U,VEL
JME0609U,XXX08
JMEM,JMEM
JMEM,MATCH3
JMEM,PGCM
JMEM,PGCU
JMEM,VEL
JMEM,XXX08
MATCH2,MATCH2
MATCH2,MATCH3
MATCH2,PGCM
MATCH2,PGCU
MATCH2,VEL
MATCH2,XXX08
MATCH3,MATCH3
MATCH3,XXX08
PGCM,PGCM
PGCM,XXX08
PGCU,PGCU
PGCU,XXX08
VEL,VEL
VEL,XXX08