Mysql SELECT为朋友系统

Question

我有这张桌子

conncections

id_user connectedto
    1       4
    2       4
    3       1
    1       5

我想做一个SELECT，它会返回所有与数字1和数字1本身有关系的数字。因此1 4 3 5为结果。

Answer 1

像

这样的东西

SELECT DISTINCT id_user as related
FROM connections
WHERE connectedto = 1
UNION
SELECT DISTINCT  connectedto as related
FROM connections
WHERE id_user = 1

删除UNION副本