ADD: 175. Combine Two Tables

SQL/README.md

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+# 175. Combine Two Tables
+
+Table: Person
+
+```sql
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| personId    | int     |
+| lastName    | varchar |
+| firstName   | varchar |
++-------------+---------+
+personId is the primary key column for this table.
+This table contains information about the ID of some persons and their first and last names.
+```
+
+Table: Address
+```sql
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| addressId   | int     |
+| personId    | int     |
+| city        | varchar |
+| state       | varchar |
++-------------+---------+
+addressId is the primary key column for this table.
+Each row of this table contains information about the city and state of one person with ID = PersonId.
+```
+
+Write an SQL query to report the first name, last name, city, and state of each person in the Person table. If the address of a personId is not present in the Address table, report null instead.
+
+Return the result table in any order.
+
+The query result format is in the following example.
+
+Example 1:
+
+```sql
+Input: 
+Person table:
++----------+----------+-----------+
+| personId | lastName | firstName |
++----------+----------+-----------+
+| 1        | Wang     | Allen     |
+| 2        | Alice    | Bob       |
++----------+----------+-----------+
+Address table:
++-----------+----------+---------------+------------+
+| addressId | personId | city          | state      |
++-----------+----------+---------------+------------+
+| 1         | 2        | New York City | New York   |
+| 2         | 3        | Leetcode      | California |
++-----------+----------+---------------+------------+
+Output: 
++-----------+----------+---------------+----------+
+| firstName | lastName | city          | state    |
++-----------+----------+---------------+----------+
+| Allen     | Wang     | Null          | Null     |
+| Bob       | Alice    | New York City | New York |
++-----------+----------+---------------+----------+
+Explanation: 
+There is no address in the address table for the personId = 1 so we return null in their city and state.
+addressId = 1 contains information about the address of personId = 2.
+```
+
+
+### INNER JOIN
+INNER JOIN (內部連接) 為等值連接，必需指定等值連接的條件，而查詢結果只會返回符合連接條件的資料。
+
+### LEFT JOIN
+可以用來建立左外部連接，查詢的 SQL 敘述句 LEFT JOIN 左側資料表 (table_name1) 的所有記錄都會加入到查詢結果中，即使右側資料表 (table_name2) 中的連接欄位沒有符合的值也一樣。
+
+### RIGHT JOIN
+相對於 LEFT JOIN，RIGHT JOIN 可以用來建立右外部連接，查詢的 SQL 敘述句 RIGHT JOIN 右側資料表 (table_name2) 的所有記錄都會加入到查詢結果中，即使左側資料表 (table_name1) 中的連接欄位沒有符合的值也一樣。

SQL/combineTwoTables.sql

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+
+SELECT Person.firstname, Person.lastName,Address.city, Address.state
+FROM Person
+Left JOIN Address
+on Person.personId=Address.personId
+
+