在計數函數中使用where子句

Question

我在創建一個2列的表。 1列包含每個月支付的金額，而另一列包含訂購當月並支付該月份的客戶數量。

select sum(paid), count(distinct customer where Order_Month = Paid_Month) 
from DataTable 
group by Paid_Month 

有沒有簡單的方法呢？

Answer 1

使用case表達：

select sum(paid), 
     count(distinct case when Order_Month = Paid_Month then customer end) 
from DataTable 
group by Paid_Month;