laravel 这个同样两张表取别名查询出错，怎么改呢？

$res = DB::table('topics')->select('topics.*', 'b.username',
            'b.avatar', 'c.username as rname', 'd.cname')
            ->where('topics.is_hidden', 0)
            ->leftJoin('users b', 'b.uid', '=', 'topics.uid')
            ->leftJoin('users c', 'c.uid', '=', 'topics.ruid')
            ->leftJoin('nodes d', 'd.node_id', '=', 'topics.node_id')
            ->orderBy('ord', 'desc')
            ->take($limit)->get();

这里错误很多：
Table 'startbbs.stb_users b' doesn't exist
stb_users 我要用到两次，肯定得取别名，怎么办呢

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$res = DB::table('topics')->select('topics.*', 'b.username',
            'b.avatar', 'c.username as rname', 'd.cname')
            ->where('topics.is_hidden', 0)
            ->leftJoin('users AS b', 'b.uid', '=', 'topics.uid')
            ->leftJoin('users AS c', 'c.uid', '=', 'topics.ruid')
            ->leftJoin('nodes AS d', 'd.node_id', '=', 'topics.node_id')
            ->orderBy('ord', 'desc')
            ->take($limit)->get();