[easy]. Second Highest Salary

Easy

注意筛选结果可能为null， 要对null进行处理

Question

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 |
+——————————-+

Approach1

Using sub-query and LIMIT clause [Accepted]
Algorithm
Sort the distinct salary in descend order and then utilize the LIMIT clause to get the second highest salary.

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

However, this solution will be judged as ‘Wrong Answer’ if there is no such second highest salary since there might be only one record in this table. To overcome this issue, we can take this as a temp table.
MySQL

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

Approach2

Using IFNULL and LIMIT clause [Accepted]
Another way to solve the ‘NULL’ problem is to use IFNULL funtion as below.
MySQL

SELECT
    IFNULL(
      (SELECT DISTINCT Salary
       FROM Employee
       ORDER BY Salary DESC
        LIMIT 1 OFFSET 1),
    NULL) AS SecondHighestSalary