sql - ユニオンを使用して最後の行の特定の列の合計を取得する方法

Question

平均の合計を取得したい

SELECT
        Employees.EmployeeID AS EMPLOYEEID,
        Employees.EMPLOYEENAMESURNAMEFORMAT AS LastFirstName,

        SUM(MAILCOUNT) MAILCOUNT,
        COUNT(*) DAYCOUNT,
        AVG(MAILCOUNT) AVERAGE 
    FROM Employees
    LEFT JOIN ST_LAWERP_DAILYMAIL_PERFORMANCE ON Employees.EmployeeID = ST_LAWERP_DAILYMAIL_PERFORMANCE.EMPLOYEEID
    WHERE Employees.ACTIVESTATUS = 1 
        AND Employees.Loginname<>''
        AND Employees.Title NOT IN ('Chauffeur','Intern','Summer Intern')
        AND ST_LAWERP_DAILYMAIL_PERFORMANCE.DATE BETWEEN '2011-01-01' AND '2015-01-01'

    GROUP BY Employees.EmployeeID,Employees.EMPLOYEENAMESURNAMEFORMAT