176-Second Highest Salary

语言：sql

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v1:

select Salary as SecondHighestSalary from Employee order by Salary desc limit 1,1

结果：未通过，因为为空时，和期待的结果null不同
v2:

SELECT (SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT 1,1) as SecondHighestSalary;