Question

父表：

adverts
- id

子表：

advertsitems
- id
- advertid
- price
- quantity

我正在尝试根据advertid计算数量，但不知怎的，我无法让它发挥作用。这就是我所拥有的：

  SELECT adverts.title AS adtitle, 
         adverts.id AS adid, 
         adverts.image AS image,
         description, 
         price, 
         shipping, 
         customs, 
         createdate
    FROM adverts 
         LEFT JOIN advertsitems 
                   ON adverts.id = advertsitems.advertid 
GROUP BY adid
ORDER BY adid DESC

我尝试添加COUNT（advertsitems.quantity），但它不算数。

Answer 1

您在没有聚合的情况下从price提取advertsitems字段。这从任意行中提取价格。以下查询将此替换为min()。您可能需要max()或avg()。

我假设select中除价格之外的所有列都来自adverts表。它们以适当的别名作为前缀。

最后，我认为你想要的是sum(quantity)，而不是count(quantity)。

  SELECT a.title AS adtitle, 
         a.id AS adid, 
         a.image AS image,
         a.description, 
         sum(ai.quantity) as quantity
         min(price) as LowestPrice, 
         a.shipping, 
         a.customs, 
         a.createdate
  FROM adverts a LEFT JOIN
       advertsitems ai
       ON a.id = ai.advertid 
  GROUP BY a.id
  ORDER BY a.id DESC

计算JOIN表中的多行

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