返回高於平均水平的行

Question

我正在嘗試查找訂單數量高於平均水平的國家/地區。這是我得到的。問題是（見圖片）的訂單的每個國家的計數是關閉的，因爲它應該是給大家

SELECT avg(NumberOrders) as avg, 
     Customers.Country, 
     NumberOrders 
    FROM Customers, 
      (SELECT COUNT(Orders.OrderId) AS NumberOrders 
      FROM Customers JOIN 
        Orders ON Customers.CustomerID = Orders.CustomerID 
     GROUP BY Customers.Country) nested 
GROUP BY Customers.Country 
    HAVING NumberOrders > avg; 

Output

Answer 1

不同。如果你的DBMS支持窗聚集（幾乎所有的除了MySQL的&訪問）：

select * 
from 
(
    SELECT Customers.Country, 
     COUNT(Orders.OrderId) AS NumberOrders, -- count per country 
     AVG(COUNT(Orders.OrderId)) OVER() AS avgOrders -- average count 
    FROM Customers 
    JOIN Orders 
    ON Customers.CustomerID = Orders.CustomerID 
    GROUP BY Customers.Country 
) nested 
WHERE NumberOrders > avgOrders 

編輯：

對於沒有的DBMS蘇pporting 窗聚集它的方式更加複雜：

SELECT Customers.Country, 
    COUNT(Orders.OrderId) AS NumberOrders -- count per country 
FROM Customers 
JOIN Orders 
    ON Customers.CustomerID = Orders.CustomerID 
GROUP BY Customers.Country 
HAVING COUNT(Orders.OrderId) > 
(select avg(NumberOrders) 
    from 
    (
     SELECT Customers.Country, 
     COUNT(Orders.OrderId) AS NumberOrders -- count per country 
     FROM Customers 
     JOIN Orders 
     ON Customers.CustomerID = Orders.CustomerID 
     GROUP BY Customers.Country 
    ) AS dt 
) 

如果DBMS支持公用表表達式這可以簡化爲：

with cte as 
(
    SELECT Customers.Country, 
     COUNT(Orders.OrderId) AS NumberOrders -- count per country 
    FROM Customers 
    JOIN Orders 
    ON Customers.CustomerID = Orders.CustomerID 
    GROUP BY Customers.Country 
) 
select * 
from cte 
WHERE NumberOrders > 
(
    select avg(NumberOrders) from cte 
)