我如何得到这个最接近的平均数？

Question

我从子查询开始，这样我就可以获得每个特定月份的总销售额。然后，我接受了每个响应并将其嵌套到一个语句中。

然后我创建了主要查询，其中添加了每个月的总数，然后除以5，给出了神秘的月平均销售额。

然后我查询了每个月的总销售额，以便将其与平均值进行比较。

 (SELECT 
    ROUND(SUM(a.total_monthly_sales + b.total_monthly_sales + c.total_monthly_sales + d.total_monthly_sales + e.total_monthly_sales) / 5,
            2) AS avg_monthly_sales,
    a.total_monthly_sales AS april,
    b.total_monthly_sales AS may,
    c.total_monthly_sales AS june,
    d.total_monthly_sales AS july,
    e.total_monthly_sales AS august
 FROM
    (SELECT 
        SUM(invoice_total) AS total_monthly_sales,
            date_format(invoice_date, '%m-%y') AS 'mm-yy'
    FROM
        `ap`.`invoices`
    WHERE
        invoice_date BETWEEN '2011-04-01' AND '2011-04-31') a
        JOIN
    (SELECT 
        SUM(invoice_total) AS total_monthly_sales,
            date_format(invoice_date, '%m-%y') AS 'mm-yy'
    FROM
        `ap`.`invoices`
    WHERE
        invoice_date BETWEEN '2011-05-01' AND '2011-05-31') b
        JOIN
    (SELECT 
        SUM(invoice_total) AS total_monthly_sales,
            date_format(invoice_date, '%m-%y') AS 'mm-yy'
    FROM
        `ap`.`invoices`
    WHERE
        invoice_date BETWEEN '2011-06-01' AND '2011-06-31') c
        JOIN
    (SELECT 
        SUM(invoice_total) AS total_monthly_sales,
            date_format(invoice_date, '%m-%y') AS 'mm-yy'
    FROM
        `ap`.`invoices`
    WHERE
        invoice_date BETWEEN '2011-07-01' AND '2011-07-31') d
        JOIN
    (SELECT 
        SUM(invoice_total) AS total_monthly_sales,
            date_format(invoice_date, '%m-%y') AS 'mm-yy'
    FROM
        `ap`.`invoices`
    WHERE
        invoice_date BETWEEN '2011-08-01' AND '2011-08-31') e)t1

如何才能显示平均值和最接近它的月份？