【leetcode Database】175.Combine Two Tables

题目：

Table: Person

+-------------+---------+| Column Name | Type    |+-------------+---------+| PersonId    | int     || FirstName   | varchar || LastName    | varchar |+-------------+---------+PersonId is the primary key column for this table.

Table: Address

+-------------+---------+| Column Name | Type    |+-------------+---------+| AddressId   | int     || PersonId    | int     || City        | varchar || State       | varchar |+-------------+---------+AddressId is the primary key column for this table.

Write a SQL query for a report that provides the following information for each person in the Person table, regardless if there is an address for each of those people:

FirstName, LastName, City, State

解析：合并两个数据表的数据，最容易想到是将两个表的PersonId用where关联起来。但是，本题要求无论Address表是否为空，都要把Person表数据检索出来。因此要用外连接来做，以下是左外连接的代码：

select FirstName,LastName,City,State from Person left outer join Address on Person.PersonId = Address.PersonId;