Very simple solution to get the n'th salary with the help of limit parameter.
select sal from employee order by sal desc limit n-1,1;
Where n is the nth number.
eg : If you want to find the 3rd max salary then you will use.
n = 3;
n-1 = 2
select sal from employee order by sal desc limit 2,1;
Hope you get the idea , By this you can get nth number of max salaries