177. Nth Highest Salary【leetcode】，第n高数值，sql，limit，offset

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                    |
+------------------------+

解题：取第n高的值，使用OFFSET和LIMIT，注意判断空的情况（将数据排序前N-1个数字跳过，取一条）

OFFSET和LIMIT

http://www.cnblogs.com/haoHaoStudyShare/p/7441315.html

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
    DECLARE M INT;
    SET M=N-1;
  RETURN (
      # Write your MySQL query statement below.
      SELECT  IFNULL((SELECT DISTINCT Salary  FROM Employee ORDER BY Salary DESC LIMIT 1 OFFSET M),NULL) AS secondHighestSalary
  );
END