选出如下结果
    image.png
    SELECT
    invoice_id,
    invoice_total,
    (SELECT AVG(invoice_total)
    FROM invoices) AS invoice_average
    invoice_total - (SELECT AVG(invoice_total)
    FROM invoices) AS difference
    FROM
    invoices
    上面写法有重复部分
    SELECT
    invoice_id,
    invoice_total,
    (SELECT AVG(invoice_total)
    FROM invoices) AS invoice_average
    invoice_total - (SELECT invoice_average) AS difference
    FROM
    INVOICES

    SELECT
    client_id,
    name,
    (SELECT SUM(invoice_total)
    FROM invoices
    WHERE client_id = c.client_id) AS total_sales,
    (SELECT AVG(invoice_total) FROM invoices) AS average,
    (SELECT total_sales) - (SELECT average) AS difference
    FROM clients c