mysql - 如何获得前3个值mysql？罗斯文数据库

Question

我正在使用示例数据库 Northwind 在 Toad for mySQL 中进行一些 SQL 练习。

此查询的目标是获取每个月的前 3 名客户。最高的意思是他们花的钱最多。我能够加入表格并得出每个客户的所有支出，但我无法获得每个月的最高值。

到目前为止，这是我的查询：

SELECT EXTRACT(MONTH FROM orders.OrderDate) AS `Month`,
orders.CustomerID,`order details`.OrderID, 
ROUND(SUM((`order details`.UnitPrice * `order details`.Quantity)*(1 - `order details`.Discount)),2) AS Cost
FROM `order details`
LEFT JOIN orders ON orders.OrderID = `order details`.OrderID
RIGHT JOIN customers ON customers.CustomerID = orders.CustomerID
GROUP BY orders.OrderID, orders.CustomerID, `Month`
ORDER BY Month DESC
LIMIT 3;

我觉得我可能需要某种循环来每月增加一次，但我不知道它的格式，但什么是可行的解决方案？

Answer 1

for month in 6 5 4 ; do mysql -u root database_name -BNe "SELECT '"$month"' AS Month,orders.CustomerID, order_details.OrderID, ROUND(SUM((order_details.UnitPrice * order_details.Quantity)*(1 - order_details.Discount)),2) AS Cost FROM order_details LEFT JOIN orders ON orders.OrderID = order_details.OrderID RIGHT JOIN customers ON customers.CustomerID = orders.CustomerID where EXTRACT(MONTH FROM orders.OrderDate)=$month GROUP BY Month,orders.OrderID, orders.CustomerID ORDER BY Month DESC ,Cost Desc limit 3"; done