mysql - SQL 使用 JOIN 查询多个表 - Northwind

Question

//个人理解，非hw赋值 所以在 MS 的示例数据库 northwind 中有表:订单 o，[订单详细信息] od，客户 c

• o 有 orderID、customerID(包括重复项)
• od 有 orderID(包括重复项)、单价、数量、折扣
• c 有 customerID，companyName

粗略地说， 我要加入

o.customerID = c.customerID; selecting companyName ///
          join on o.orderID = od.orderID; selecting unitprice, quantity, discount

我的最终目标是

 sum(q (up - d)) AS 'Order Total' group by od.orderID then
                  sum(Order Total) group by companyName(?)

不过，我的主要问题是不知道如何/加入什么才能正确加入。

提前致谢

Answer 1

在 SQL Fiddle 上查看您的场景

SELECT comp.`company_name` AS 'company',COUNT(DISTINCT o.id_sales_order) AS 'total_orders',SUM(`unit_price`) AS 'grand_total'
FROM sales_order AS o
LEFT JOIN sales_order_item AS od ON od.fk_sales_order = o.id_sales_order
LEFT JOIN customer AS c ON c.id_customer = o.fk_customer
LEFT JOIN company AS comp ON comp.id_company = c.fk_company_id
GROUP BY comp.`company_name`

希望这就是你要找的东西