Question

我正在尝试编写一个查询，该查询将生成商家列表，然后计算他们拥有的库存物品数量以及每个库存物品的订单数量。商家，库存物品和订单都在不同的表格中。我期待回归看起来像这样：

Site    | Inventory Items | Orders
London  | 7282            | 1234996
Glasgow | 9829            | 734949

我可以通过此查询几乎立即获得网站和库存计数列表：

SELECT M.name AS 'Site',
       COUNT(II.idInventoryItems) AS 'Inventory Items',
FROM Merchants M
JOIN
  (SELECT idMerchants,
          idInventoryItems
   FROM InventoryItems) AS II ON II.idMerchants = M.idMerchants
GROUP BY M.name

我可以通过非常相似的查询获得网站和订单列表。但是当我将两者结合起来时，查询会无限期挂起，我必须取消查询。这是我一直在使用的完整查询：

SELECT M.name AS 'Site',
       COUNT(II.idInventoryItems) AS 'Inventory Items',
       COUNT(O.idOrders) AS 'Orders'
FROM Merchants M
JOIN
  (SELECT idMerchants,
          idInventoryItems
   FROM InventoryItems) AS II ON II.idMerchants = M.idMerchants
JOIN
  (SELECT idMerchants,
          idOrders
   FROM Orders) AS O ON O.idMerchants = M.idMerchants
GROUP BY M.name

我正在尝试用合理快速的查询实现的目标吗？如果是这样，我做错了什么？

Answer 1

试试这个。将计数移动到派生表将为您提供连接所需的数据，并允许mysql使用idMerchants上的索引进行优化（如果它需要它用于性能目的）。然后，您将加入两个派生表，其中行数明显减少，这也将提高性能。

SELECT M.name AS 'Site',
   II.count AS 'Inventory Items',
   O.count as 'Orders'
FROM Merchants M
JOIN
  (SELECT idMerchants, COUNT(idInventoryItems) AS 'count'
   FROM InventoryItems GROUP BY idMerchants) AS II ON II.idMerchants = M.idMerchants
JOIN
  (SELECT idMerchants,
      COUNT(idOrders) AS 'count'
   FROM Orders GROUP BY idMerchants) AS O ON O.idMerchants = M.idMerchants

连接多个表和计数行时挂起查询

1 个答案: