Question

我正在使用SQL Server 2008.

我有一个需要稍微调整的查询。当我计算与网站相关的品牌时，它非常适合，但当我尝试计算与品牌相关的模型时，网站数量会发生变化。

表格是

SITES
SiteID // pk
SiteName

BRANDS
BrandID // pk
SiteID // fk
Brand

MODELS
ModelID // pk
BrandID // fk

此查询生成与网站关联的正确的TotalBrands数量：

        SELECT      S.SiteID,
                    S.SiteName,
                    COUNT(B.BrandID) AS TotalBrands
        FROM        Sites S
                    LEFT JOIN Brands B
                        ON S.SiteID = B.SiteID
        GROUP BY    S.SiteID, S.SiteName

当我向查询添加更多内容时，TotalBrands是不准确的。

        SELECT      S.SiteID,
                    S.SiteName,
                    COUNT(B.BrandID) AS TotalBrands,
                    COUNT(M.ModelID) AS TotalModels
        FROM        Sites S
                    LEFT JOIN Brands B
                        ON S.SiteID = B.SiteID
                    LEFT JOIN Models M
                        ON B.BrandID = M.BrandID
        GROUP BY    S.SiteID, S.SiteName

我想要包括所有网站，无论他们是否有品牌。我想要计算所有品牌是否有型号。我想要计算所有模型。

我的查询有什么问题？

Answer 1

正如评论中所说的那样，你在上一个JOIN上乘以结果。因此，您要么进行DISTINCT计数，要么先进行分组。

SELECT      S.SiteID,
            S.SiteName,
            COUNT(B.BrandID) AS TotalBrands,
            SUM(M.Models) AS TotalModels
FROM        Sites S
            LEFT JOIN Brands B
                ON S.SiteID = B.SiteID
            LEFT JOIN ( SELECT BrandId, COUNT(*) Models 
                        FROM Models
                        GROUP BY BrandId) M
                ON B.BrandID = M.BrandID
GROUP BY    S.SiteID, S.SiteName

或者

SELECT      S.SiteID,
            S.SiteName,
            COUNT(DISTINCT B.BrandID) AS TotalBrands,
            COUNT(M.ModelId) AS TotalModels
FROM        Sites S
            LEFT JOIN Brands B
                ON S.SiteID = B.SiteID
            LEFT JOIN Models M
                ON B.BrandID = M.BrandID
GROUP BY    S.SiteID, S.SiteName

SQL - 从三个表计数的查询中的轻微错误

1 个答案: