mySQL查询的意外结果

Question

我的问题是我没有得到我需要的正确输出。

我有three tables

产品

id  title
1   aaa
2   bbb
3   ccc

订单

id  order_date
1   01-20-2016
2   02-27-2016
3   03-29-2016

ORDER_DETAILS

id  prod_id   order_id  quantity
1   1           1           1   
2   3           1           1
3   2           2           2
4   1           2           1
5   3           2           1
6   2           3           1
7   1           3           2

不期望的输出

title   date        Quantity
aaa     January         2
aaa     February        4
aaa     March           3

我期待的输出

title   date        quantity
aaa     January         1
aaa     Febuary         1
aaa     March           2
bbb     January         0
bbb     Febuary         2
bbb     March           1
ccc     January         1
ccc     February        1
ccc     March           0

这是我目前的查询。

SELECT products.title, DATE_FORMAT(orders.order_date, '%M') as date, SUM(order_details.quantity) as quantity 
FROM products, orders, order_details 
WHERE products.id = order_details.product_id and orders.id = order_details.order_id
GROUP BY date

有人可以帮助我获取expected output吗？提前谢谢。

Answer 1

您的GROUP BY缺少标题字段，因此只使用了月份

尝试

GROUP BY title, date

Answer 2

您只按日期对结果进行分组，因此例如对于1月，它将总结以下几行：

aaa     January         1
bbb     January         0
ccc     January         1

，输出将是

aaa     January         2 (sum of 1+0+1)

使用

GROUP BY title, date