leetcode176 第二高的薪水 Second Highest Salary

编写一个 SQL 查询，获取 Employee 表中第二高的薪水（Salary） 。

 创建表和数据：

drop table Employee;
Create table If Not Exists Employee (Idint, Salary int);

insert into Employee (Id, Salary) values('1', '100');
insert into Employee (Id, Salary) values('2', '200');
insert into Employee (Id, Salary) values('3', '300');

解法：

1.表自连接

用表的自连接，构造偏序关系。再找次序的最大值，就一定是第二高的薪水。同时，max在没有元组输入时，会返回NULL。如在表中的元组少于2个时。

SELECT MAX(e1.salary) as SecondHighestSalary
FROM Employee e1 JOIN Employee e2
WHERE e1.salary < e2.salary;

2.子查询

子查询方法。用子查询找出最大值，再排除最大值的结果中，求最大值。必须要借助于MAX函数。

select max(salary) as SecondHighestSalary
from Employee
where salary !=(select max(salary) from Employee);

select max(E.Salary) as SecondHighestSalary
from Employee as E
where E.Salary < (
select max(Salary)
from Employee 
);

select max(E1.Salary) as SecondHighestSalary
from Employee as E1
where 1 = (
    select count(*)
    from Employee as E2
    where E1.Salary < E2.Salary
);

不用MAX函数的方法

SELECT
(SELECT DISTINCT
        Salary FROM Employee  ORDER BY Salary DESC    LIMIT 1 OFFSET 1) AS SecondHighestSalary;