筛选员工，筛选员工的工资需要大于他所在部门的平均工资
    SELECT
    FROM employees e
    WHERE salary > (
    SELECT AVG(salary)
    FROM employees
    WHERE *office_id = e.office_id
    )

    练习
    image.png
    SELECT
    client_id
    FROM
    invoices i
    WHERE invoice_total > (
    SELECT AVG(invoice_total)
    FROM invoices
    WHERE client_id = i.client_id
    )