我有以下表格，我想获取purchase_order和他的order_quantity以及每个purchase_order的接收数量之和。我知道如何从一张表中求出数量的总和，但从多张表中，这让我很困惑。。。
mysql> select * from purchase_order;
+-------------------+-------------------------+-------+---------------------+
| purchase_order_id | purchase_order | cost | created_on |
+-------------------+-------------------------+-------+---------------------+
| 1 | Dell Computer 000001256 | 10000 | 2015-02-19 22:14:52 |
| 2 | HP Computer 000001256 | 50000 | 2015-02-19 22:14:52 |
+-------------------+-------------------------+-------+---------------------+
2 rows in set (0.00 sec)
mysql> select * from purchase_order_detail;
+--------------------------+-------------------+---------+------------------+
| purchase_order_detail_id | purchase_order_id | item_id | ordered_quantity |
+--------------------------+-------------------+---------+------------------+
| 1 | 1 | 279 | 100 |
| 2 | 1 | 286 | 100 |
| 3 | 2 | 279 | 200 |
| 4 | 2 | 286 | 300 |
+--------------------------+-------------------+---------+------------------+
4 rows in set (0.00 sec)
mysql> select * from delivery_order;
+-------------------+--------------------------+-------------------+---------------------+
| delivery_order_id | purchase_order_detail_id | recieved_quantity | recieved_on |
+-------------------+--------------------------+-------------------+---------------------+
| 1 | 1 | 50 | 2015-02-19 22:22:51 |
| 2 | 2 | 50 | 2015-02-19 22:24:59 |
| 3 | 1 | 50 | 2015-02-19 22:34:14 |
| 4 | 3 | 70 | 2015-02-20 11:11:31 |
| 5 | 4 | 150 | 2015-02-20 11:11:31 |
| 6 | 3 | 90 | 2015-02-20 11:12:20 |
| 7 | 4 | 100 | 2015-02-20 11:12:20 |
| 8 | 3 | 40 | 2015-02-20 11:12:55 |
| 9 | 4 | 50 | 2015-02-20 11:12:55 |
+-------------------+--------------------------+-------------------+---------------------+
到目前为止，我有这个查询，但id没有返回正确的记录。。
SELECT po.purchase_order_id, SUM(pod.ordered_quantity) AS Sum_of_ordered_quantity, SUM(dor.recieved_quantity) AS Sum_of_recieved_quantity
FROM purchase_order AS po
INNER JOIN purchase_order_detail AS pod ON po.purchase_order_id = pod.purchase_order_id
INNER JOIN delivery_order AS dor ON dor.purchase_order_detail_id = pod.purchase_order_detail_id
GROUP BY po.purchase_order_id
它返回这个，
+-------------------+-------------------------+--------------------------+
| purchase_order_id | Sum_of_ordered_quantity | Sum_of_received_quantity |
+-------------------+-------------------------+--------------------------+
| 1 | 300 | 150 |
| 2 | 1500 | 500 |
+-------------------+-------------------------+--------------------------+
您可以在问题中看到，purchase_order_id 1有200个已订购数量和150个已接收数量，而purchase_order_id 2有500个已订购的数量和500个已接收的数量。
请尝试此代码：
SELECT po.purchase_order_id,
SUM(pod.ordered_quantity) AS Sum_of_ordered_quantity,
(SELECT `mySelect`.`desired_sum` FROM
(SELECT B.`purchase_order_id` AS myID, SUM( `A`.`recieved_quantity` ) AS desired_sum
FROM `delivery_order` AS A
LEFT JOIN `purchase_order_detail` AS B ON A.purchase_order_detail_id = B.purchase_order_detail_id
GROUP BY B.`purchase_order_id` ) AS mySelect
WHERE `mySelect`.`myID` = `po`.`purchase_order_id`) AS Sum_of_received_quantity
FROM purchase_order AS po
INNER JOIN purchase_order_detail AS pod ON po.purchase_order_id = pod.purchase_order_id
GROUP BY po.purchase_order_id
这在对具有不同联接条件的多个多对多表执行聚合和函数时很常见。一种方法是使用相关子查询来获取聚合值，然后进行联接。
select
po.purchase_order_id,
pod.Sum_of_ordered_quantity,
do.Sum_of_received_quantity
from purchase_order po
join
(
select purchase_order_id,sum(ordered_quantity) as Sum_of_ordered_quantity
from purchase_order_detail
group by purchase_order_id
)pod on pod.purchase_order_id = po.purchase_order_id
join
(
select
t1.purchase_order_id,
sum(t2.recieved_quantity) as Sum_of_received_quantity
from purchase_order_detail t1
join delivery_order t2 on t1.purchase_order_detail_id = t2.purchase_order_detail_id
group by t1.purchase_order_id
)do on do.purchase_order_id = po.purchase_order_id
DEMO