查询第二高薪水，若不存在则返回null

查询第二高薪水，若不存在则返回null

在leetcode上有这样的一道题：

查询中第二高的薪水值，不存在则返回null

SELECT 

(SELECT DISTINCT salary FROM employee GROUP BY salary DESC LIMIT 1,1) AS SECOND

或者

SELECT (
  SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT 1 OFFSET 1

)AS SECOND

或者

SELECT 
CASE COUNT(1)
     WHEN 1 THEN NULL
     ELSE MIN(salary) END AS SecondHighestSalary
 FROM 
(SELECT salary FROM employee  GROUP BY salary ORDER BY salary DESC LIMIT 2) t