[MySQL][LeetCode][Easy] 176. Second Highest Salary

心得：

找出該資料表內第二高薪水的筆數，若無則回傳null。

問題：

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 second highest salary is 200. If there is no second highest salary, then the query should return null.

答案：

1. Sub Query

   # Write your MySQL query statement below
   SELECT  MAX(a.Salary) AS SecondHighestSalary
   FROM    Employee AS a
   WHERE   a.Salary < (SELECT  MAX(z.Salary)
                       FROM    Employee AS z)

2. DISTINCT > 不重複

   # Write your MySQL query statement below
   SELECT
   (
       SELECT DISTINCT Salary
       FROM            Employee
       ORDER BY        Salary DESC
       LIMIT           1, 1
   ) AS SecondHighestSalary

3. GROUP BY > 不重複

   # Write your MySQL query statement below
   SELECT
   (
       SELECT          Salary
       FROM            Employee
       GROUP BY        Salary
       ORDER BY        Salary DESC
       LIMIT           1, 1
   ) AS SecondHighestSalary