176. 第二高的薪水

#取出前大的子表，再找出最小的
SELECT 
        IF(COUNT(Salary)=2, MIN(Salary), NULL)   #if(ex1,ex2,ex3),如果ex1为真，则输出结果为ex2，否则为ex3  
AS  
        SecondHighestSalary  #取别名
FROM (SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT 2) #取出最大的前两个
AS
        nMAX;  #取别名