如何正確連接這兩個表格？

Question

我有這兩個表

客戶：

id | name    | address 
---+---------------------+------------------------- 
1 | company 1   | some address information 
2 | company 2   | another address 
3 | yet another company | no address here 

訂單：

id | customer_id | date 
---+-------------+--------- 
1 | 2   | 20151209 
2 | 2   | 20151211 
3 | 3   | 20151210 
4 | 1   | 20151223 

現在我希望得到一個結果表與左邊每個客戶和訂單量在的任意時間段右側。 例如，給定此期間爲20151207 < =日期< = 20151211，所得到的表應該是這樣的：

name    | orders count 
--------------------+------------- 
company 1   | 0 
company 2   | 2 
yet another company | 1 

注：日期= 20151207裝置12月7日2015年

如何加入他們？

Answer 1

SELECT c.name，COUNT（CASE WHEN（（o.date BETWEEN 20151207 AND 20151211）OR（o.date ISNULL））THEN o.customer_id END）AS「Total Sales」FROM customers AS c LEFT JOIN orders o ON c.id == o.customer_id GROUP BY c.name