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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 | +---------------------+
解题思路：
注意两点：首先如果没有第二高的工资，应该返回NULL，如果直接使用查询会查询为空，而不会返回NULL，所以需要将此查询作为子查询。
第二点是找第二高的工资，是工资多少排第二，而不是工资排名第二，所以需要distinct修饰。
select
(select distinct Salary from Employee order by Salary desc limit 1,1)
as SecondHighestSalary;