select i.*,u.username,f.username as friendusername from friendinfo i,userinfo u,userinfo f where i.userid = u.userid and i.friendid=f.userid
少了筛选条件 select i.*,u.username,f.username as friendusername from friendinfo i,userinfo u,userinfo f where i.userid = u.userid and i.friendid=f.userid and i.userid = 1000
from friendinfo i,userinfo u,userinfo f
where i.userid = u.userid
and i.friendid=f.userid
select i.*,u.username,f.username as friendusername
from friendinfo i,userinfo u,userinfo f
where i.userid = u.userid
and i.friendid=f.userid
and i.userid = 1000