Delete almost duplicates in a table - sql

Question

I have a table where strings 1 and 2 are almost duplicate - they have the same values but in reverse order. How can I delete these duplicates?
+--------+-------+
| COL_1 | COL_# |
+--------+-------+
| a1 | b1 |
| b1 | a1 | <- same as 1stline but in reversed order, needs to be removed
| a2 | b2 |
| a3 | b3 |
| b3 | a3 |<-- also is duplicate of string above, one of these 2str need
+--------+-------+ to be removed
expected result:
+--------+-------+
| COL_1 | COL_# |
+--------+-------+
| b1 | a1 |
| a2 | b2 |
| a3 | b3 |
+--------+-------+
or
+--------+-------+
| COL_1 | COL_# |
+--------+-------+
| a1 | b1 |
| a2 | b2 |
| a3 | b3 |
+--------+-------+

Answer 1

Do you mean like this :
DELETE(
select e.COL_#, f.COL_1
from example as e
join example as f on e.COL_# = f.COL_1 and e.COL_# < f.COL_1 )

Answer 2

If col_1 and col_# cannot be null, you can use LEASTand GREATEST to keep one row per combination:
delete from tbl
where rowid not in
(
select min(rowid) -- one rowid per combination to keep
from tbl
group by least(col_1, col_#), greatest(col_1, col_#)
);

Answer 3

I think it is a bit complicated to achieve it with a delete query.
Maybe it is OK for you to have such a select query first:
select A,B from (
select A,
B,
ROW_NUMBER() over (PARTITION BY ORA_HASH(A) * ORA_HASH(B) ORDER BY A) as RANK
FROM <your_table_name>
) where RANK = 1;
You could save the result of this query as a new table with CREATE TABLE AS SELECT ...
And then you simply DROP your old table.

Answer 4

One possible solution is:
DELETE FROM T
WHERE (COL_1, "COL_#") IN (SELECT COL_1, "COL_#"
FROM (SELECT ROWNUM AS RN, t1.COL_1, t1."COL_#"
FROM T t1
INNER JOIN T t2
ON t2."COL_#" = t1.COL_1 AND
t2.COL_1 = t1."COL_#")
WHERE RN / 2 <> TRUNC(RN / 2));
Note that COL_# must be quoted in Oracle as # is not a legal character in an unquoted identifier.
dbfiddle here

Answer 5

This doesn't seem so complicated:
delete t
where not (col1 < col2 or
not exists (select 1
from t t2
where t2.col1 = t.col2 and
t2.col2 = t.col1
)
);
Or:
delete t
where col1 > col2 and
exists (select 1
from t t2
where t2.col1 = t.col2 and
t2.col2 = t.col1
);