SELECT
    (SELECT DISTINCT
            Salary
        FROM
            Employee
        ORDER BY Salary DESC
        LIMIT 1 OFFSET 1) AS SecondHighestSalary
;

CREATE TABLE employee
(employee_id SERIAL PRIMARY KEY
,salary INT NOT NULL
);

INSERT INTO employee VALUES
(1,100);

 SELECT DISTINCT
             Salary
         FROM
             Employee
         ORDER BY Salary DESC
         LIMIT 1 OFFSET 1;
Empty set (0.01 sec)

 SELECT
     (SELECT DISTINCT
             Salary
         FROM
             Employee
         ORDER BY Salary DESC
         LIMIT 1 OFFSET 1) AS SecondHighestSalary
 ;
+---------------------+
| SecondHighestSalary |
+---------------------+
|                NULL |
+---------------------+