Northwind SQL每個客戶的總訂單數

Question

我在Customers表中添加了一列TotalOrders。

ALTER TABLE customers ADD TotalOrders INT NULL 

我試圖找到每個客戶的訂單總數和值添加此列，但我想不通，我需要總結究竟

INSERT INTO customers (TotalOrders) SELECT SUM(...) 

Answer 1

我想你應該在這裏使用更新。您可以彙總每個客戶在orders表中的總訂單數，然後使用此信息更新customers表。

UPDATE t1 
SET TotalOrders = t2.TotalOrders 
FROM customers t1 
INNER JOIN 
(
    SELECT CustomerID, COUNT(*) AS TotalOrders 
    FROM orders 
    GROUP BY CustomerID 
) t2 
    ON t1.CustomerID = t2.CustomerID 

Answer 2

INSERT INTO customers (TotalOrders) 
VALUES(SELECT SUM(Orders.Total) 
FROM Orders INNER JOIN Customers ON Orders.CustomerId= 
Customer.Id Group By Customer.Id) WHERE Customer.Id =Orders.CustomerId 

什麼