MySQL 查询 Select、SUM、LEFT JOIN答案

【问题标题】：MySQL query Select, SUM, LEFT JOINMySQL 查询 Select、SUM、LEFT JOIN
【发布时间】：2015-02-22 15:17:59
【问题描述】：

名为 stock 的表 1 包含列 Items
名为 Products_sold 的表 2 包含 商品、数量
名为 products_purchased 的表 3 包含 Items, Quantity

我需要获取三列的 sql 查询，例如

（产品）（表 1 中的所有项目）
（已售出的产品）（product_sold 表中每个项目的数量总和）
（购买的产品）（product_purchased 表中每个项目的数量总和）

我有查询，但它不能正常工作

  Select stock.Items as Products, 
         SUM(Products_sold.Quantity) as [Products Sold], 
         SUM(Products_purchased.Quantity) as [Products purchased]
  From 
         (
           (Stock LEFT JOIN products_purchased ON stock.Items=products_purchased.Items) 
           LEFT JOIN products_sold ON stock.Items=products_sold.Item
         )
  Group By stock.Items

【问题讨论】：

编辑您的问题并尝试解决此问题。

标签： mysql select group-by sum left-join

【解决方案1】：

试试这个：

SELECT S.Items as Products, 
       COALESCE(PS.Quantity, 0) as [Products Sold], 
       COALESCE(PP.Quantity, 0) as [Products purchased]
From Stock S 
LEFT JOIN (SELECT Items, SUM(Quantity) AS Quantity 
           FROM products_purchased 
           GROUP BY Items
         ) AS PP ON S.Items = PP.Items 
LEFT JOIN (SELECT Items, SUM(Quantity) AS Quantity 
           FROM products_sold 
           GROUP BY Items
         ) AS PS ON S.Items = PS.Items;

【讨论】：