correct sql query to simplify relationship (mysql) - mysql

Question

I have the following table:
table A
id emp emp_dst
1 a b
2 a d
3 b c
4 b a
5 c d
6 d a
7 d b
8 d c
my sql query should return me the following simplified table since a = b equals b = a
table B
emp emp_dst
a b
a d
b c
d b
d c
but I have no idea how to do this in an sql query in MYSQL,
try revising expressions with UNION but the results are wrong

Answer 1

An alternative that suits my personal preferences better...
(Based on your comment that the id in the results was not relevant.)
SELECT
CASE WHEN emp <= emp_dst THEN emp ELSE emp_dst END AS emp,
CASE WHEN emp <= emp_dst THEN emp_dst ELSE emp END AS emp_dst
FROM
yourTable
GROUP BY
1, 2
ORDER BY
1, 2
If you want an id, then you can add MIN(id). Just note that the id found may actually have the two values the other way around.
An alternative that uses a LEFT JOIN rather than GROUP BY.
SELECT
yourTable.*
FROM
yourTable
LEFT JOIN
yourTable AS reflection
ON reflection.emp_dst = yourTable.emp
AND reflection.emp = yourTable.emp_dst
AND reflection.id <> yourTable.id
WHERE
(reflection.id IS NULL)
OR (yourTable.emp < reflection.emp_dst)
OR (yourTable.emp = reflection.emp_dst AND yourTable.id < reflection.id)
ORDER BY
yourTable.emp,
yourTable.emp_dst
(The last OR is only needed if a table can have 'a', 'a', and it appear twice.)
Note: This may benefit from having two indexes...
CREATE INDEX yourTable_e_ed_id ON yourTable( emp, emp_dst, id );
CREATE INDEX yourTable_ed_e_id ON yourTable( emp_dst, emp, id );