Question

假设这些表：

群组 :(身份证，标题）：{1,G1}, {2,G2}, {3,G3}, {4, G4}

类别 :( Id，标题）：{1, Cat1}, {2, Cat2}, {3, Cat3}, {4, Cat4}

产品 :( Id，GroupId，CategoryId，Name）：

{1, 1, 1, G1C1P1},
{2, 1, 2, G1C2P2},
{3, 1, 2, G1C2P3},
{4, 2, 2, G2C2P4},
{5, 2, 2, G2C2P5},
{6, 3, 1, G3C1P6},
{7, 3, 3, G3C3P7}

所以我需要的是上述值的任何类别的计数是：

Group  Category  Count
----------------------
 G1     Cat1      1
 G1     Cat2      2
 G1     Cat3      0
 G1     Cat4      0
 G2     Cat1      0
 G2     Cat2      2
 G2     Cat3      0
 G2     Cat4      0
 G3     Cat1      1
 G3     Cat2      0
 G3     Cat3      1
 G3     Cat4      0
 G4     Cat1      0
 G4     Cat2      0
 G4     Cat3      0
 G4     Cat4      0

我试试这个：

SELECT 
        [GR].[Title] AS [Group],
        COUNT([PR].[Id]) AS [Count],
        [CA].[Title]

FROM [dbo].[Group] AS [GR]
FULL OUTER JOIN [dbo].[Product] AS [PR] ON [GR].[Id] = [PR].[GroupId]
FULL OUTER JOIN [dbo].[Category] AS [CA] ON [PR].[CategoryId] = [CA].[Id]
GROUP BY [CA].[Title], [GR].[Title];
GO

但这不是确切的，所以你的建议是什么？

Answer 1

您需要CROSS JOIN两个表（Group和Category）创建所有可能的组类别组合，然后LEFT JOIN创建Product表：

SELECT 
    gr.Title     AS [Group],
    ca.Title,
    COUNT(pr.Id) AS [Count]
FROM 
        dbo.[Group] AS gr
    CROSS JOIN 
        dbo.Category AS ca 
    LEFT OUTER JOIN 
        dbo.Product AS pr 
            ON  pr.GroupId = gr.Id
            AND pr.CategoryId = ca.Id 
GROUP BY 
    gr.Title,
    ca.Title 
ORDER BY
    gr.Title,
    ca.Title ;

或以这种方式（Products表中的第一个GROUP BY然后加入派生表）：

SELECT 
    gr.Title             AS [Group],
    ca.Title,
    COALESCE(pr.cnt, 0)  AS [Count]
FROM 
        dbo.[Group] AS gr
    CROSS JOIN 
        dbo.Category AS ca 
    LEFT OUTER JOIN 
        ( SELECT 
              GroupId,
              CategoryId,
              COUNT(*) AS cnt
          FROM
              dbo.Product 
          GROUP BY 
              GroupId,
              CategoryId
        ) AS pr
            ON  pr.GroupId = gr.Id
            AND pr.CategoryId = ca.Id 
ORDER BY
    gr.Id,
    ca.Id ;

如果Group(Title)和Category(Title)列是唯一的，则查询是等效的（排序除外）。

Answer 2

试试这个：

SELECT 
        [GR].[Title] AS [Group],
        [CA].[Title],
        COUNT([PR].[Id]) AS [Count]

FROM [dbo].[Product] AS [PR]
FULL OUTER JOIN [dbo].[Group] AS [GR] ON [PR].[GroupId] = [GR].[Id]
FULL OUTER JOIN [dbo].[Category] AS [CA] ON [PR].[CategoryId] = [CA].[Id]
GROUP BY [GR].[Title],[CA].[Title];
GO

计算SQL中每类产品的计数

2 个答案: