从products表内查询比生菜（Lettuce）贵的产品
    SELECT *
    FROM products
    WHERE unit_price > (
    SELECT unit_price
    FROM products
    WHERE product_id = 3
    )

    在employees表中获取比平均工资高的员工
    SELECT *
    FROM employees
    WHERE salary > (
    SELECT
    AVG(salary)
    FROM
    employees
    )