176. Second Highest Salary

方法１，子查询

SELECT MAX(Salary) as SecondHighestSalary 
FROM Employee 
WHERE Salary < (SELECT MAX(Salary) FROM Employee)

方法２，构建临时表

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

临时表为什么可以把空值变为 NULL 还是不明白。