[SQL]LeetCode176. 第二高的薪水 | Second Highest Salary

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

编写一个 SQL 查询，获取 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

105ms

1 # Write your MySQL query statement below
2 select (
3   select distinct Salary from Employee order by Salary Desc limit 1 offset 1
4 )as SecondHighestSalary

106ms

1 # Write your MySQL query statement below
2 select max(salary) as SecondHighestSalary
3 from employee
4 where salary < (select max(salary) from employee)

108ms

1 # Write your MySQL query statement below
2 # select distinct Salary SecondHighestSalary from Employee order by Salary DESC limit 1,1
3 select ifNull((select distinct Salary from Employee order by Salary Desc limit 1,1),null) as SecondHighestSalary

110ms

1 # Write your MySQL query statement below
2 SELECT IFNULL( (SELECT DISTINCT salary
3 FROM Employee
4 ORDER BY salary DESC
5 LIMIT 1,1),NULL) AS SecondHighestSalary;