[SQL Question] Selecting distinct rows having field value pairs on semantics

Hi all,
I am looking for a way of selecting records from a
table comprising of pairing fields having unique
semantics, where the pair of values of lets say 'left'
and 'right' and another pair having values of 'right'
and 'left' is considered as duplicates as they have
the same meaning.

Below is my table structure.
create table ppi_edges_tf
(
tf_id1 char(6)not null,
tf_id2 char(6)not null,
primary key(tf_id1,tf_id2)
);

I would like to create a query on this table that will
contain only unique [tf_id1 and tf_id2] combination is
unique semantically.
for example looking at a select rows of the table
tf_id1 | tf_id2
-------+--------
T00111 | T00111
T00111 | T00112
T00111 | T01400
T00111 | T05015
T00112 | T00111

The second record (T00111 | T00112) and the fifth
record(T00112 | T00111) have the same pairing meaning
and should be considered duplicate in my case and only
one for the records (either one) should be contained
in the desired resultset of unique values.
The desired resultset could be something like this
tf_id1 | tf_id2
-------+--------
T00111 | T00111
T00111 | T00112
T00111 | T01400
T00111 | T05015

Allan.

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