https://leetcode.com/problems/nth-highest-salary/
ATTENTION:limit 子句只能接受int常量,不能接受运算式
CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
SET N = N - 1;
RETURN (
# Write your MySQL query statement below.
select DISTINCT Salary from Employee Order by Salary DESC limit N,1
);
END