LeetCode177.Nth Highest Salary

Table: Employee

+-------------+------+
| Column Name | Type |
+-------------+------+
| id          | int  |
| salary      | int  |
+-------------+------+
id is the primary key column for this table.
Each row of this table contains information about the salary of an employee.

 Write an SQL query to report the nth highest salary from the Employee table. If there is no nth highest salary, the query should report null.
The query result format is in the following example.

Input: 
Employee table:
+----+--------+
| id | salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+
n = 2
Output: 
+------------------------+
| getNthHighestSalary(2) |
+------------------------+
| 200                    |
+------------------------+

CREATE FUNCTION getNthHighestSalary(N IN NUMBER) RETURN NUMBER IS

result NUMBER;

BEGIN

    /* Write your PL/SQL query statement below */

    WITH  real_query  AS (

        select salary as SecondHighestSalary from (

            select salary, dense_rank() OVER (ORDER BY salary DESC) RANK from Employee

        ) where RANK=N

    )

    SELECT  SecondHighestSalary into result FROM real_query

    union

    select null FROM dual

    WHERE NOT EXISTS(SELECT 1 FROM real_query);

    RETURN result;

END;

Ref:

https://medium.com/jimmy-wang/oracle%E5%9F%BA%E6%9C%AC%E4%BF%AE%E7%B7%B4-pl-sql-select-into-6c66313782ce