How to remove opposite rows from SQL table.

Question

I have the following SQL table:

A|B
---
w|x
x|w
y|z
z|y

Can I construct a query which will yield the following result:

A|B
---
w|x
y|z

To summarize, I would like to treat the two columns as an unordered set, such that (a,b) == (b,a).

Answer 1

You could try the following:

SELECT LEAST(a,b) a, GREATEST(a,b) b
FROM t
GROUP BY LEAST(a,b), GREATEST(a,b)

---

With the following test-table t:

CREATE TABLE t ( a VARCHAR(1), b VARCHAR(1) );

INSERT INTO t VALUES ('w','x'),('x','w'),('y','z'),('z','y');

it returns:

w  x
y  z

Using LEAST and GREATEST also makes sure that w x is returned instead of x w.

Answer 2

The "best" code depends on the database, but following is dbms-agnostic:

SELECT      t.A,
            t.B
FROM        my_table t
LEFT JOIN   my_table t2
        ON  t.A = t2.B
        AND t.B = t2.A
        AND t.A < t.B
WHERE       t2.A IS NULL