https://leetcode.com/problems/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 second highest salary is 200. If there is no second highest salary, then the query should return null.
首先MySQL不支持top语法，要用limit代替。
select Salary from Employee order by Salary desc limit 1,1
这样写的话如果不存在，select出来是空，题目需要是null(掀桌。
Input: {"headers": {"Employee": ["Id", "Salary"]}, "rows": {"Employee": [[1, 100]]}}
Output: {"headers": ["Salary"], "values": []}
Expected: {"headers": ["SecondHighestSalary"], "values": [[null]]}
于是就需要在外面再包一层。为什么要加distinct呢，是因为不加过不了这个case：
+----+--------+ | Id | Salary | +----+--------+ | 1 | 100 | | 2 | 100 | +----+--------+
最终解法:
select (select distinct Salary from Employee order by Salary desc limit 1,1) as Salary;
[LeetCode][SQL]Second Highest Salary
原文：http://www.cnblogs.com/Liok3187/p/4518650.html