查找第二高的薪水

題目：編寫一個 SQL 查詢，獲取 Employee 表中第二高的薪水（Salary）

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

| Id | Salary |

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

| 1 | 100 |

| 2 | 200 |

| 3 | 300 |

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

例如上述 Employee 表，SQL查詢應該返回 200 作爲第二高的薪水。如果不存在第二高的薪水，

那麼查詢應返回 null。

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

| SecondHighestSalary |

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

| 200 |

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

SQL:

select min(salary) as "SecondHighestSalary"
  from (select salary, row_number() over(order by salary desc) rn
          from (select distinct Salary from Employee))
 where rn != 1
   and rn <= 2

rn!=1是爲了防止Table中只有一條數據。

當where條件不成立時，min(salary)的結果是空值，聚合函數爲空的時候返回null

具體題目詳情見：https://leetcode-cn.com/problems/second-highest-salary/