Add solution 176 [sql]

Author: yanglbme

README.md

@@ -35,6 +35,7 @@ Complete [solutions](https://github.com/doocs/leetcode/tree/master/solution) to
 | 141 | [Linked List Cycle](https://github.com/doocs/leetcode/tree/master/solution/141.Linked%20List%20Cycle) | `Linked List`, `Two Pointers` |
 | 155 | [Min Stack](https://github.com/doocs/leetcode/tree/master/solution/155.Min%20Stack) | `Stack`, `Design` |
 | 175 | [Combine Two Tables](https://github.com/doocs/leetcode/tree/master/solution/175.Combine%20Two%20Tables) | `SQL` |
+| 176 | [Second Highest Salary](https://github.com/doocs/leetcode/tree/master/solution/176.Second%20Highest%20Salary) | `SQL` |
 | 189 | [Rotate Array](https://github.com/doocs/leetcode/tree/master/solution/189.Rotate%20Array) | `Array` |
 | 198 | [House Robber](https://github.com/doocs/leetcode/tree/master/solution/198.House%20Robber) | `Dynamic Programming` |
 | 203 | [Remove Linked List Elements](https://github.com/doocs/leetcode/tree/master/solution/203.Remove%20Linked%20List%20Elements) | `Linked List` |

solution/176.Second Highest Salary/README.md

@@ -0,0 +1,73 @@
+## 第二高的薪水
+### 题目描述
+
+编写一个 SQL 查询，获取 `Employee` 表中第二高的薪水（Salary） 。
+```
++----+--------+
+| Id | Salary |
++----+--------+
+| 1  | 100    |
+| 2  | 200    |
+| 3  | 300    |
++----+--------+
+```
+
+例如上述 `Employee` 表，SQL查询应该返回 `200` 作为第二高的薪水。如果不存在第二高的薪水，那么查询应返回 `null`。
+```
++---------------------+
+| SecondHighestSalary |
++---------------------+
+| 200                 |
++---------------------+
+```
+
+### 解法
+从小于最高薪水的记录里面选最高即可。
+
+```sql
+# Write your MySQL query statement below
+select max(Salary) SecondHighestSalary from Employee where Salary < (Select max(Salary) from Employee)
+
+```
+
+#### Input
+```json
+{
+    "headers": {
+        "Employee": [
+            "Id",
+            "Salary"
+        ]
+    },
+    "rows": {
+        "Employee": [
+            [
+                1,
+                100
+            ],
+            [
+                2,
+                200
+            ],
+            [
+                3,
+                300
+            ]
+        ]
+    }
+}
+```
+
+#### Output
+```json
+{
+    "headers": [
+        "SecondHighestSalary"
+    ],
+    "values": [
+        [
+            200
+        ]
+    ]
+}
+```