SELECT a.p_key, b.p_key
FROM test AS a, test AS b
WHERE
a.p_key != b.p_key -- skip self
AND a.p_key < b.p_key -- symmetry breaker
-- missing extra condition to skip previously fetched results

a.p_key p.b_key
1   2
1   3

2   3* (already linked to a.p_key = 1)
2   4

3   4* (already linked to a.p_key = 2)
3   5

a.p_key p.b_key
1   2
1   3

2   4

3   5