text: # Write your MySQL query statement below
  
  # 这样是有问题的 如果表中只有一条记录将返回空而不是NULL
  select Salary as SecondHighestSalary from Employee
  order by Salary desc
  limit 1, 1;
  
  select IFNULL((select distinct Salary from Employee order by Salary desc limit 1, 1),
      null) as SecondHighestSalary
  
  # 另一种解法 也挺有意思的
  SELECT max(Salary)
  FROM Employee
  WHERE Salary < (SELECT max(Salary) FROM Employee)
  
  