Question

I am using SQL Server 2008.

I have a query that needs a little tweaking. It counts perfectly well when I count The Brands that are associated with the Sites, but when I try to count the Models that are associated with the brands, the number of sites changes.

The tables are

SITES
SiteID // pk
SiteName

BRANDS
BrandID // pk
SiteID // fk
Brand

MODELS
ModelID // pk
BrandID // fk

This query yields the correct number of TotalBrands associated with a site:

        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

When I add a little bit more to the query, the TotalBrands is inaccurate.

        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

I want to include all sites, whether they have Brands or not. I want to count all Brands whether they have Models or not. I want to count all Models.

What’s wrong with my query?

Answer 1

As was said in the comments, you are multipliying results on your last JOIN. So you etiher do a DISTINCT count, or do the grouping before.

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

Or

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