176. Second Highest Salary

Write a SQL query to get the second highest salary from the Employee table.

+----+--------+
| Id | Salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+

For example, given the above Employee table, the query should return 200 as the second highest salary. If there is no second highest salary, then the query should return null.

+---------------------+
| SecondHighestSalary |
+---------------------+
| 200                 |
+---------------------+

Solution:

### 1.
SELECT DISTINCT Salary
FROM Employee 
UNION ALL (SELECT NULL AS Salary)
ORDER BY Salary DESC LIMIT 1 OFFSET 1;

### 2.
SELECT MAX(Salary) AS SecondHighestSalary
FROM Employee 
WHERE Salary NOT IN
(SELECT MAX(Salary) FROM Employee);