Question

我有3张桌子

Products
[id, name]

Purchases
[id, product_id, quantity, date]

Sales
[id, product_id, quantity, date]

我想列出每种产品的剩余数量的所有产品。

我已尝试使用以下查询，但它无效。

SELECT pr.id, pr.name, (sum(pu.quantity))-sum(s.quantity) as quantity 
FROM `products` pr 
join purchase pu ON pu.product_id = pr.id
left join sales s ON s.product_id = pr.id
GROUP BY pr.id

我知道上面的查询会为purchase和sales创建重复的行，因为我们将这两个表与产品一起加入。

请帮我写出正确的查询以查找剩余的产品数量

Answer 1

您需要在join之前汇总。一种方法明确在from子句中：

SELECT pr.id, pr.name,
       (coalesce(pu.sumq, 0) - coalesce(s.sumq, 0)) as quantity 
FROM `products` pr left join
     (SELECT pu.product_id, SUM(pu.quantity) as sumq
      FROM purchase pu
      GROUP BY pu.product_id
     ) pu
     ON pu.product_id = pr.id left join
     (SELECT s.product_id, SUM(s.quantity) as sumq
      FROM sales s 
      GROUP BY s.product_id
     ) s
     ON s.product_id = pr.id
GROUP BY pr.id, pr.name;

Answer 2

请改用IFNULL。因为 12 + null = null

SELECT pr.id, pr.name, 
sum(ifnull(pu.quantity,0))-sum(ifnull(s.quantity,0)) as quantity 
FROM `products` pr 
left join purchase pu ON pu.product_id = pr.id
left join sales s ON s.product_id = pr.id
GROUP BY pr.id,pr.name

由于您在结果集中添加了pr.name，因此您还需要将其放在GROUP BY子句中。

mysql - 将一个表与另外两个表连接起来

2 个答案: