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 |
解法
一开始的思路,用max()函数查询两次
SELECT MAX(e.Salary) AS SecondHighestSalary
FROM Employee e
WHERE e.Salary < (
SELECT MAX(Salary)
FROM Employee
);
但是效率很差,做了两次查询
一次结果中查询第二条
select DISTINCT Salary from Employee ORDER BY Salary DESC limit 1 offset 1;
结果还是不行,DISTINCT 似乎会使返回值为null的时候返回空,所以外层需要嵌套一层
select (select DISTINCT Salary from Employee ORDER BY Salary DESC limit 1 offset 1) as SecondHighestSalary;