177. 第N高的薪水

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
RETURN (
# Write your MySQL query statement below.
-- 取前N个最大数的最小数
SELECT 
    IF(COUNT(Salary)=N, MIN(Salary), NULL)     
AS  
    getNthHighestSalary  
FROM (SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT N) 
AS  
    mMax  
);
END