Question 1. Get Nth highest salary

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
  RETURN (
      SELECT RS.Salary
      FROM (
          SELECT @RNUM := @RNUM + 1 as rank, t1.Salary
          FROM
          (
            SELECT distinct Salary
            FROM Employee
            ORDER BY Salary desc
          ) t1,
          ( SELECT @RNUM := 0 ) R
      ) RS
      WHERE RS.rank = N    
  );
END

Question 2. Not exists example

# Write your MySQL query statement below
SELECT c.Name Customers
FROM Customers c
WHERE NOT EXISTS (SELECT 'X' FROM Orders o WHERE c.Id = o.CustomerId);