leetcode-176. second highest salary

Write a SQL query to get the second highest salary from the Employee table.

+----+--------+
| Id | Salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+

For example, given the above Employee table, the query should return 200 as the second highest salary. If there is no second highest salary, then the query should return null.

+---------------------+
| SecondHighestSalary |
+---------------------+
| 200                 |
+---------------------+

思路：
　　按薪水salary 倒序排序，然后从第一个开始，取下一个。
　　
一开始我的SQL(Wrong Answer)

  SELECT DISTINCT Salary as SecondHighestSalary
    FROM Employee
ORDER BY Salary DESC
   LIMIT 1 OFFSET 1

忽略了如果表中salary的值只有一种的情况，这样就没有the second highest salary，输出 null.
正确写法：

SELECT IFNULL(
      (SELECT DISTINCT Salary
       FROM Employee
       ORDER BY Salary DESC
        LIMIT 1 OFFSET 1),
       NULL) AS SecondHighestSalary

IFNULL
　　这儿用到了 MySQL 的 IFNULL(expr1,expr2)函数，如果 expr1 不是 NULL，IFNULL() 返回 expr1，否则它返回 expr2。
　　

leetcode-176. second highest salary

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