MySQL 8.0

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CREATE TABLE merged_table (source CHAR(1), destination CHAR(1));
INSERT INTO merged_table VALUES
(    'a',        'b'),
(    'b',        'a'),
(    'c',        'd'),
(    'd',        'c');
SELECT * FROM merged_table;


source
destination


a
b

b
a

c
d

d
c


SELECT t1.source t1_source, t1.destination t1_destination,
       t2.source t2_source, t2.destination t2_destination
FROM merged_table t1
JOIN merged_table t2 ON t1.source = t2.destination
                    AND t1.destination = t2.source;


t1_source
t1_destination
t2_source
t2_destination


b
a
a
b

a
b
b
a

d
c
c
d

c
d
d
c


SELECT t1.source t1_source, t1.destination t1_destination,
       t2.source t2_source, t2.destination t2_destination,
       t1.source > t2.source AS third_condition
FROM merged_table t1
JOIN merged_table t2 ON t1.source = t2.destination
                    AND t1.destination = t2.source;


t1_source
t1_destination
t2_source
t2_destination
third_condition


b
a
a
b
1

a
b
b
a
0

d
c
c
d
1

c
d
d
c
0


SELECT t1.source t1_source, t1.destination t1_destination,
       t2.source t2_source, t2.destination t2_destination
FROM merged_table t1
JOIN merged_table t2 ON t1.source = t2.destination
                    AND t1.destination = t2.source
                    AND t1.source > t2.source;


t1_source
t1_destination
t2_source
t2_destination


b
a
a
b

d
c
c
d


DELETE t1.*
FROM merged_table t1
JOIN merged_table t2 ON t1.source = t2.destination
                    AND t1.destination = t2.source
                    AND t1.source > t2.source;
SELECT * FROM merged_table;


source
destination


a
b

c
d