第二高的薪水

方法一：
SELECT
    IFNULL(
        (
            SELECT DISTINCT
                Salary
            FROM
                Employee
            ORDER BY
                Salary DESC
            LIMIT 1 OFFSET 1
        ),
        NULL
    ) AS SecondHighestSalary;

方法二：
SELECT
    MAX(Salary) AS SecondHighestSalary
FROM
    Employee
WHERE
    Salary < (
        SELECT
            MAX(Salary)
        FROM
            Employee
    );