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

    上面这段查询会形成一个新表，可以用来做查询
    这叫视图
    SELECT
    FROM (
    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
    ) AS sales_summary
    WHERE total_sales IS NOT NULL