问题：
编写一个sql查询，从employee表中获取第n个最高工资。

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

例如，在上面的employee表中，n=2的第n个最高工资是200。如果没有第n个最高工资，那么查询应该返回null。

+------------------------+
| getNthHighestSalary(2) |
+------------------------+
| 200                    |
+------------------------+

我的问题：

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
  RETURN(

    SELECT NT.Salary as CONCAT('getNthHighestSalary','(',N,')')
    FROM (SET @i := 0;
          SELECT (@i := @i + 1) as Index, Id, Salary
          FROM Employee
          ORDER BY Salary DESC) AS NT
    WHERE NT.Index = N);
END

错误：

Line 5: SyntaxError: near '('getNthHighestSalary','(',N,')')
    FROM (SET @i := 0;
          SELECT (@i :='