LeetCode 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.

注意相同的salary算一位，而且limit以及offset不能用表达式，至少我写的时候是不行的。

一开始不知道如何声明一个变量，结果硬是写了这个方法：

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INTBEGIN  RETURN (      select(select a.salary from(select distinct b.salary from Employee b union all select max(c.salary) from Employee c)a order by a.salary desc limit 1 offset N)  );END

后来在discuss看到了如何声明变量：

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INTBEGINdeclare M int;set M = N - 1;  RETURN (      select distinct salary from Employee order by salary desc limit 1 offset M  );END