SELECT b.UserName AS name1,
c.UserName AS name2
FROM b LEFT JOIN a ON b.id1=a.UserID
LEFT JOIN a ON b.id2=a.UserID
c.UserName AS name2
FROM b LEFT JOIN a ON b.id1=a.UserID
LEFT JOIN a ON b.id2=a.UserID
调试欢乐多
left join a as A on A.userid = B.id1
left join a as C on C.userid = B.id2