本SQL问题是 #176 Second Highest Salary
本题目是求第二高薪的问题。很容易就会想到用LIMIT和OFFSET来写:
select
distinct(p.salary) as SecondHighestSalary
from employee as p
order by p.salary DESC
limit 1 offset 1
但是这个SQL是'Wrong'，因为当数据只有一条时,上面的SQL返回是空而非系统要求的NULL
而SQL要返回NULL可以直接select 一个空就行了，所以以上的SQL改进为：
select (
select distinct(p.salary)
from employee as p
order by p.salary DESC
limit 1 offset 1
) as SecondHighestSalary
这样就可以Accepted
