How To remove duplicate rows
DELETE e1 FROM EMPLOYEE e1, EMPLOYEE e2 WHERE e1.name = e2.name AND e1.id > e2.id;
for more details

1.
find Highest salary select MAX(Salary) from Employee;
2.
Find Second Highest Salary
ELECT MAX(Salary) FROM Employee WHERE Salary NOT IN (SELECT MAX(Salary) FROM Employee )
3.
Find Nth Highest Salary
SELECT * /*This is the outer query part */
FROM Employee Emp1
WHERE (N-1) = ( /* Subquery starts here */
SELECT COUNT(DISTINCT(Emp2.Salary))
FROM Employee Emp2
WHERE Emp2.Salary > Emp1.Salary)

we can use limit as well
SELECT Salary FROM Employee
ORDER BY Salary DESC LIMIT n-1,1