Question

此刻我在使用SQL时遇到问题。

的示例数据库

这就是我现在拥有的：

SELECT 
    Categories.CategoryID, 
    Categories.CategoryName, 
    COUNT(Products.CategoryID) AS CountProduct
FROM 
    Categories
INNER JOIN 
    Products ON Products.CategoryID = Categories.CategoryID
GROUP BY
    CategoryName
ORDER BY 
    CategoryName ASC

输出：

CategoryID  CategoryName    CountProduct
-----------------------------------------
1           Beverages           12
2           Condiments          12
3           Confections         13
4           Dairy Products      10
5           Grains/Cereals      7
6           Meat/Poultry        6
7           Produce             5
8           Seafood             12

这：

SELECT 
    Categories.CategoryID, 
    Categories.CategoryName, 
    COUNT(OrderDetails.ProductID) AS CountOrder
FROM 
    Categories
LEFT JOIN 
    Products ON Products.CategoryID = Categories.CategoryID
LEFT JOIN 
    OrderDetails ON OrderDetails.ProductID = Products.ProductID
GROUP BY
    CategoryName
ORDER BY 
   CategoryName ASC

输出：

CategoryID  CategoryName    CountOrder
------------------------------------------------
1           Beverages           93
2           Condiments          49
3           Confections         84
4           Dairy Products      100
5           Grains/Cereals      42
6           Meat/Poultry        50
7           Produce             33
8           Seafood             67

我只想要这个结果：

CategoryID  CategoryName    CountProduct    CountOrder
-------------------------------------------------------
1           Beverages           12              93
2           Condiments          12              49
3           Confections         13              84
4           Dairy Products      10              100
5           Grains/Cereals      7               42
6           Meat/Poultry        6               50
7           Produce             5               33
8           Seafood             12              67

我试图将两个查询合并为一个，但是得到了错误的结果。

谢谢

Answer 1

使用标量子查询

SELECT Categories.CategoryID, Categories.CategoryName, Count(Products.CategoryID) as CountProduct,
(select Count(OrderDetails.ProductID) from orderdetails where OrderDetails.ProductID = Products.ProductID) as CountOrder
FROM Categories
INNER JOIN Products On Products.CategoryID = Categories.CategoryID
Group By CategoryName
ORDER BY CategoryName ASC

Answer 2

由于这是一次学习课程，似乎他们正在尝试向您介绍非重复计数，因此请使用以下查询，

SELECT Categories.CategoryID, Categories.CategoryName, COUNT(DISTINCT Products.PRODUCTID) as ProductCout, Count(OrderDetails.ProductID ) as CountOrder
FROM Categories
LEFT JOIN Products On Products.CategoryID = Categories.CategoryID
LEFT JOIN OrderDetails ON OrderDetails.ProductID = Products.ProductID
Group By Categories.CategoryID, Categories.CategoryName

SQL合并两个不同表的联接结果

2 个答案: