Add files via upload

Author: wave-gbt

Database/LeetCode-176-Second-Highest-Salary.md

@@ -0,0 +1,54 @@
+---
+title: LeetCode-176. Second Highest Salary
+date: 2017-11-07 14:22:31
+tags: [LeetCode,mysql]
+permalink: leetcode-second-height-salary
+copyright: true
+password:
+top:
+---
+
+Write a SQL query to get the second highest salary from the `Employee` table.
+<!-- more -->
+
+```
++----+--------+
+| 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。
+　　
+