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notes/Leetcode-Database 题解.md

+<!-- GFM-TOC -->
+* [175. Combine Two Tables](#175-combine-two-tables)
+* [未完待续...](#未完待续)
+<!-- GFM-TOC -->
+
+
+# 175. Combine Two Tables
+
+https://leetcode.com/problems/combine-two-tables/description/
+
+## Describe
+
+Table: Person
+
+```html
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| PersonId    | int     |
+| FirstName   | varchar |
+| LastName    | varchar |
++-------------+---------+
+PersonId is the primary key column for this table.
+```
+
+Table: Address
+
+```html
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| AddressId   | int     |
+| PersonId    | int     |
+| City        | varchar |
+| State       | varchar |
++-------------+---------+
+AddressId is the primary key column for this table.
+```
+
+取出 FirstName, LastName, City, State 数据，而不管一个用户有没有填地址信息。
+
+## SQL Schema
+
+```sql
+CREATE TABLE Person ( PersonId INT, FirstName VARCHAR ( 255 ), LastName VARCHAR ( 255 ) );
+CREATE TABLE Address ( AddressId INT, PersonId INT, City VARCHAR ( 255 ), State VARCHAR ( 255 ) );
+TRUNCATE TABLE Person;
+INSERT INTO Person ( PersonId, LastName, FirstName )
+VALUES
+    ( 1, 'Wang', 'Allen' );
+TRUNCATE TABLE Address;
+INSERT INTO Address ( AddressId, PersonId, City, State )
+VALUES
+    ( 1, 2, 'New York City', 'New York' );
+```
+
+## Solution
+
+使用左外连接。
+
+```sql
+SELECT FirstName, LastName, City, State
+FROM Person AS p LEFT JOIN address AS a
+ON p.PersonId = a.PersonId;
+```
+
+# 未完待续...