编写一个 SQL 查询，获取 Employee 表中第 n 高的薪水（Salary）。

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Id Salary
1 100
2 200
3 300
---- --------

例如上述 Employee 表，n = 2 时，应返回第二高的薪水 200。如果不存在第 n 高的薪水，那么查询应返回 null。

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getNthHighestSalary(2)
200
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--①
CREATE FUNCTION getNthHighestSalary(N IN NUMBER) RETURN NUMBER IS
result NUMBER;
BEGIN
    /* Write your PL/SQL query statement below */
    select(
SELECT SecondHighestSalary FROM ( select Salary SecondHighestSalary,ROWNUM num from(select distinct Salary from Employee order by Salary desc)) WHERE num=N) SecondHighestSalary into result from dual;

    RETURN result;
END;
--② 使用分析函数
CREATE FUNCTION getNthHighestSalary(N IN NUMBER) RETURN NUMBER IS
result NUMBER;
BEGIN 
    select max(salary) into result from(
    select salary,dense_rank() over(order by salary desc) as rn from employee)
    where rn = n;
    RETURN result;
END;