得到平均數

Question

我有一個Shippers表和Orders表。要獲得每個託運人發送的訂單數，我使用以下語句：

SELECT Shippers.ShipperName,COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders 
LEFT JOIN Shippers 
ON Orders.ShipperID=Shippers.ShipperID 
GROUP BY ShipperName; 

它工作正常。根據以上聲明的結果，我怎樣才能得到每個託運人發送的訂單的平均數平均數？

Answer 1

嘗試使用子查詢

SELECT myT.*, AVG(NumberOfOrders) FROM(

    SELECT Shippers.ShipperName,COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders 
    LEFT JOIN Shippers 
    ON Orders.ShipperID=Shippers.ShipperID 
    GROUP BY ShipperName; 

) myT 

Answer 2

您可以通過使用AVG（）函數實現這一目標：

SELECT Shippers.ShipperName, AVG(Orders.OrderID) AS AvgNumberOfOrders 
FROM Orders 
LEFT JOIN Shippers 
ON Orders.ShipperID=Shippers.ShipperID 
GROUP BY ShipperName; 

的更多信息：http://www.w3schools.com/sql/sql_func_avg.asp

Answer 3

一種方法是一個子查詢：

SELECT AVG(NumberOfOrders) 
FROM (SELECT s.ShipperName, COUNT(o.OrderID) AS NumberOfOrders 
     FROM Orders o LEFT JOIN 
      Shippers s 
      ON o.ShipperID = s.ShipperID 
     GROUP BY s.ShipperName 
    ) t; 

然而，一個更簡單的方式就是自己做了分工：

SELECT COUNT(o.OrderID)/COUNT(DISTINCT s.ShipperName) 
FROM Orders o LEFT JOIN 
    Shippers s 
    ON o.ShipperID = s.ShipperID; 

注：兩個合理的假設是：每個託運人都有自己的ID，所有託運人有至少一個數量級。如果是這樣，你可以簡化爲：

SELECT COUNT(o.OrderID)/COUNT(DISTINCT l.ShipperId) 
FROM Orders o;