176

要考虑为空和重复的情况，ifnull和distinct关键字的使用

select IFNULL((select distinct Salary from Employee order by Salary desc limit 1,1),null) 
as SecondHighestSalary

177

注意在sql语句中不能使用表达式N-1是不能使用的

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
  if N<=0 then 
    RETURN(select min(Salary) from Employee);
  else 
   set N = N-1;
    RETURN (
        # Write your MySQL query statement below.
        select ifnull((select distinct Salary from Employee order by Salary desc limit N,1),null) as NthHighestSalay 
    );
  end if;
END