176. 第二高的薪水

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                 |
+---------------------+

+----+--------+
| Id | Salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+

例如上述 Employee 表，SQL查询应该返回 200 作为第二高的薪水。如果不存在第二高的薪水，那么查询应返回 null。

+---------------------+
| SecondHighestSalary |
+---------------------+
| 200                 |
+---------------------+

方法1：使用子查询和LIMIT子句

算法：按降序对不同的工资进行排序，然后利用该LIMIT子句获得第二高的工资。

1 SELECT DISTINCT
2     Salary AS SecondHighestSalary
3 FROM
4     Employee
5 ORDER BY Salary DESC
6 LIMIT 1 OFFSET 1

但是，如果没有这样的第二高薪，这个解决方案将被判定为“错误答案”，因为此表中可能只有一条记录。为了解决这个问题，我们可以将其作为临时表。

1 SELECT
2     (SELECT DISTINCT
3             Salary
4         FROM
5             Employee
6         ORDER BY Salary DESC
7         LIMIT 1 OFFSET 1) AS SecondHighestSalary
8 ;

方法2：使用IFNULL和LIMIT子句

解决‘NULL‘问题的另一种方法是使用IFNULL如下功能。

1 SELECT
2     IFNULL(
3       (SELECT DISTINCT Salary
4        FROM Employee
5        ORDER BY Salary DESC
6         LIMIT 1 OFFSET 1),
7     NULL) AS SecondHighestSalary

原文地址：https://www.cnblogs.com/strengthen/p/9720894.html