mysql 虚拟列连接问题

Question

select @rd := @rd+1 as rownum, b.qishu from (select @rd:=1, qishu,(one+two) as xxx from lotto_ppp HAVING xxx = 3 order by qishu DESC) b
上面这条语句执行的结果是

select @rd := @rd+1 as rownum, a.qishu from (select @rd:=0, qishu,(one+two) as xxx from lotto_ppp HAVING xxx = 3 order by qishu DESC) a
这条语句执行的结果为

为什么最终执行的这条语句连接不起来
SELECT g1.,g2. FROM (select @rd := @rd+1 as rownum, b.qishu from (select @rd:=1, qishu,(one+two) as xxx from lotto_ppp HAVING xxx = 3 order by qishu DESC) b) AS g1
LEFT JOIN ( select @rd := @rd+1 as rownum, a.qishu from (select @rd:=0, qishu,(one+two) as xxx from lotto_ppp HAVING xxx = 3 order by qishu DESC) a ) AS g2
ON g1.rownum = g2.rownum;

g2表的东西都是空的