1、176题目，Second Highest Salary，https://leetcode.com/problems/second-highest-salary/#/description
Write a SQL query to get the second highest salary from the Employee table.
+----+--------+ | Id | Salary | +----+--------+ | 1 | 100 | | 2 | 200 | | 3 | 300 | +----+--------+
For example, given the above Employee table, the query should return 200 as the second highest salary. If there is no second highest salary, then the query should return null.
+---------------------+ | SecondHighestSalary | +---------------------+ | 200 | +---------------------+
select (
select distinct Salary from Employee order by Salary Desc limit 1 offset 1
)as second；
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177. Nth Highest Salary
Write a SQL query to get the nth highest salary from the Employee table.
+----+--------+ | Id | Salary | +----+--------+ | 1 | 100 | | 2 | 200 | | 3 | 300 | +----+--------+
For example, given the above Employee table, the nth highest salary where n = 2 is 200. If there is no nth highest salary, then the query should return null.
+------------------------+ | getNthHighestSalary(2) | +------------------------+ | 200 | +------------------------+
CREATE FUNCTION getNthHighestSalary(N INT)
RETURNS INT
BEGIN
DECLARE M INT;
SET M=N-1;
RETURN (
SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT M, 1]
);
END