Add solution 175 [sql]

Author: yanglbme

README.md

@@ -34,6 +34,7 @@ Complete [solutions](https://github.com/doocs/leetcode/tree/master/solution) to
 | 136 | [Single Number](https://github.com/doocs/leetcode/tree/master/solution/136.Single%20Number) | `Hash Table`, `Bit Manipulation` |
 | 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` |
 | 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/175.Combine Two Tables/README.md

@@ -0,0 +1,97 @@
+## 组合两个表
+### 题目描述
+
+表1: `Person`
+```
++-------------+---------+
+| 列名         | 类型     |
++-------------+---------+
+| PersonId    | int     |
+| FirstName   | varchar |
+| LastName    | varchar |
++-------------+---------+
+PersonId 是上表主键
+```
+
+表2: `Address`
+```
++-------------+---------+
+| 列名         | 类型    |
++-------------+---------+
+| AddressId   | int     |
+| PersonId    | int     |
+| City        | varchar |
+| State       | varchar |
++-------------+---------+
+AddressId 是上表主键
+```
+
+编写一个 SQL 查询，满足条件：无论 person 是否有地址信息，都需要基于上述两表提供 person 的以下信息：
+```
+FirstName, LastName, City, State
+```
+
+### 解法
+题意中说无论 `person` 是否有地址信息，都要查出来，因此，使用左外连接查询。注意使用 `on` 关键字。
+
+```sql
+# Write your MySQL query statement below
+select a.FirstName, a.LastName, b.City, b.State from Person a left join Address b on a.PersonId = b.PersonId;
+
+```
+
+#### Input
+```json
+{
+    "headers": {
+        "Person": [
+            "PersonId",
+            "LastName",
+            "FirstName"
+        ],
+        "Address": [
+            "AddressId",
+            "PersonId",
+            "City",
+            "State"
+        ]
+    },
+    "rows": {
+        "Person": [
+            [
+                1,
+                "Wang",
+                "Allen"
+            ]
+        ],
+        "Address": [
+            [
+                1,
+                2,
+                "New York City",
+                "New York"
+            ]
+        ]
+    }
+}
+```
+
+#### Output
+```json
+{
+    "headers": [
+        "FirstName",
+        "LastName",
+        "City",
+        "State"
+    ],
+    "values": [
+        [
+            "Allen",
+            "Wang",
+            null,
+            null
+        ]
+    ]
+}
+```