分组两列（使用逗号不能按预期工作）

Question

如何分组两列？

我当前的查询：

SELECT 
MIN(DATE_FORMAT(t.date_booked, '%Y-%m-%d')) as date,
MIN(DATE_FORMAT(i.inv_date, '%Y-%m-%d')) as invdate,

FROM Hours as t

LEFT JOIN Proj as p
ON t.pid = p.id

LEFT JOIN inv_c as c
ON t.pid = c.pid

LEFT JOIN inv as i
ON c.pid = i.id


WHERE t.pid = "2005"


GROUP BY MONTH(t.date_booked), MONTH(i.inv_date)
ORDER BY t.date_booked, i.inv_date ASC

这是

的输出

|       date        |       invdate     |
|   2015-01-02      |       2015-02-10  |
|   2015-01-02      |       2015-02-15  |   
|   2015-01-05      |       2015-02-10  |
|   2015-01-05      |       2015-02-15  |   
.....

这应该是它应该做的吗？因为它看起来只是对invdate列进行分组，

我还尝试添加＆＃39; AND＆＃39;在分组GROUP BY MONTH(t.date_booked) AND MONTH(i.inv_date)

上

但它只输出第一行

|       date        |       invdate     |
|   2015-01-02      |       2015-02-10  |

我想要的输出是这样的：

|       date        |       invdate     |
|   2015-01-02      |       2015-02-10  |
|   2015-01-05      |       2015-02-15  |

最好的方法是什么？