LeetCode——Nth Highest Salary

write a sql query to get the nth highest salary from the employee table.

+----+--------+
| id | salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+
for example, given the above employee table, the nth highest salary where n = 2 is 200. if there is no nth highest salary, then the query should return null.

+------------------------+
| getnthhighestsalary(2) |
+------------------------+
| 200                    |
+------------------------+

此题相较于second highest salary做了一些改进：

• 创建mysql function;
• 需要判断传入参数的合理性.

因此，对代码改动如下所示：

create function getnthhighestsalary(n int) returns int
begin
  declare p int default n-1;
  if (p<0) then
    return null;
  else
  return (
      # write your mysql query statement below.
      select ifnull(
            (
                select distinct salary 
                from employee 
                order by salary desc 
                limit p,1)
            ,null)
          as secondhighestsalary   
  );
  end if;
end