如何獲得列中字段的計數，從而消除該字段中的重複條目？

Question

我正試圖爲每位客戶計算總髮票。我從中提取信息的表格是一個交易表格，對於每個發票號碼（[Document No_]）可能有許多條目。這個查詢的結果是給了我每個客戶的行數，但沒有明確計數的發票。我不確定如何寫這個。

SELECT top 20 a.[Sell-to Customer No_], 
       b.[Name], 
       sum(a.[Amount])as 'Total Sales Amount', 
       avg(a.Amount) as 'Average per Purchase', 
       count(a.[Document No_]) as 'Total Invoices' 
FROM [Sales Invoice Line] a left outer join [Customer] b 
ON a.[Sell-to Customer No_] = b.[No_] 
WHERE [Posting Group] like 'PAR' 
and a.[Shipment Date] >= '10-01-2013' 
GROUP BY a.[Sell-to Customer No_], b.[Name] 

Answer 1

嘗試使用：的

count(distinct...) 

代替

count(...)