SQL INNER JOIN 使用 CASE 和 3 个表

Question

我有以下表格，我只想显示可用的类型，同时计算该类别中的产品。

类型

Type_Id	Name
1	Candy
2	Chocolate Bar

产品

Product_Id	Name	Type_Id	AvailabilityId
1	Chocolate Name 1	1	1
2	Chcoolate Name 1	2	2
3	Candy Name 1	2	2

可用性

Availability_Id	Name
1	Available
2	Reserved
3	Sol d

期望的结果

Type_Id	Name	TotalAvailable
1	Candy	1
2	Chocolate	2

Answer 1

您可以使用带有内部联接的内部查询来实现此目的。这是 SQL fiddle: http://sqlfiddle.com/#!9/42bd7f/2

Answer 2

从基本的开始 JOIN between all three tables, on the related columns. Note use of table aliases t - 类型，p - 产品，a - 可用性

SELECT * 
FROM   Types t 
         INNER JOIN Products p ON p.Type_Id = t.Type_Id
         INNER JOIN Availability a ON a.Availability_Id = p.Availability_Id

接下来添加一个 WHERE 子句以按可用性状态过滤结果：

SELECT * 
FROM   Types t 
        INNER JOIN Products p ON p.Type_Id = t.Type_Id
        INNER JOIN Availability a ON a.Availability_Id = p.Availability_Id
WHERE  a.Name IN ('Available', 'Reserved')

结果：

Type_Id | Name          | Product_Id | Name           | Type_Id | Availability_Id | Availability_Id | Name     
------: | :------------ | ---------: | :------------- | ------: | --------------: | --------------: | :--------
      1 | Candy         |          1 | Chocolate Name |       1 |               1 |               1 | Available
      2 | Chocolate Bar |          2 | Chcoolate Name |       2 |               2 |               2 | Reserved 
      2 | Chocolate Bar |          3 | Candy Name     |       2 |               2 |               2 | Reserved

最终 COUNT(*) 匹配的总行数，按类别分组（即 [Type].[Name]）

SELECT t.Type_Id
       , t.Name AS Type_Name
       , COUNT(*) AS Total_Products
FROM   Types t 
        INNER JOIN Products p ON p.Type_Id = t.Type_Id
        INNER JOIN Availability a ON a.Availability_Id = p.Availability_Id
WHERE  a.Name IN ('Available', 'Reserved')
GROUP BY t.Type_Id
         , t.Name

Type_Id | Type_Name     | Total_Products
------: | :------------ | -------------:
      1 | Candy         |              1
      2 | Chocolate Bar |              2

SQL Fiddle

SQL INNER JOIN 使用 CASE 和 3 个表

SQL INNER JOIN using CASE and 3 tables

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