SELECT (
SELECT Salary
FROM (
SELECT DISTINCT Salary, DENSE_RANK() OVER(ORDER BY Salary DESC) rank
FROM Employee
) T
WHERE T.rank = 2
) "SecondHighestSalary"
FROM DUAL
*Oracle은 공백을 null값으로 처리하기 때문에 null을 출력하기 위해 dual 테이블 사용
SELECT
IFNULL (NULL,
(SELECT DISTINCT Salary
FROM Employee
ORDER BY Salary DESC
LIMIT 1 OFFSET 1)) AS SecondHighestSalary
IFNULL(expression, alt_value)