得到平均数

Question

我有一个Shippers表和Orders表。要获得每个托运人发送的订单数，我使用以下语句：

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。根据以上声明的结果，我怎样才能得到每个托运人发送的订单的平均数平均数？

Answer 1

尝试使用子查询

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

您可以通过使用AVG（）函数实现这一目标：

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

一种方法是一个子查询：

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，一个更简单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：两个合理的假设是：每个托运人都有自己的ID，所有托运人有至少一个数量级。如果是这样，你可以简化为：

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