SCCM/ConfigMgr : Count of Workstation by Active Directory Site

SELECT
    R.AD_Site_Name0,
    Count(SMS_Installed_Sites0) AS 'Count'
From
    v_R_System R
    JOIN v_RA_System_SMSInstalledSites SIS ON R.ResourceID = SIS.ResourceID
WHERE
    R.AD_Site_Name0 = 'Default-First-Site-Name'
Group By
    R.AD_Site_Name0

Thanks a lot to Garth Jones (Weblog) for this tricks