显示不同的元组，无论列顺序如何

这样尝试

SELECt DISTINCT       LEAST(s1.country, s2.country) c1,       GREATEST(s1.country, s2.country) c2  FROM battles b1 JOIN battles b2    ON b1.battlename = b2.battlename   AND b1.ship <> b2.ship JOIN ships s1    ON b1.ship = s1.name JOIN ships s2    ON b2.ship = s2.nameHAVINg c1 <> c2

输出：

|      C1 | C2 ||---------|---------------|| Germany | Great Britain ||   Japan |USA |

这是 SQLFiddle 演示