leetcode 176. Second Highest Salary

方法1、

SELECT  IFNULL(MAX(Salary), NULL) SecondHighestSalaryFROM  EmployeeWHERE Salary <  (SELECT    MAX(Salary)  FROM    Employee)

方法2、

    SELECT DISTINCT  (Salary) AS SecondHighestSalaryFROM  EmployeeUNIONSELECT  NULLORDER BY SecondHighestSalary DESCLIMIT 1, 1