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                 |

+---------------------+

从表里面查询出第二大数值

首先要用distinct去重，因为第二大的数值可能是有重复的，然后从高到低排序，取第二个数据就是。但是要注意的是可能表里面只有一条数据，如果想要返回NULL的话，可以在最外层再加一个select查询

select 
 (
    select distinct salary from Employee order by salary desc limit 1,1
 ) as SecondHighestSalary