【LeetCode之MySQL】175. Combine Two Tables_mysql 175. combine two tables

问题：

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

解决办法：

通过左关联 表A left join 表2 on 相同字段 

左关联（Left Join）就是以左边的表为准，只要左边的表有数据就输出，不管右边的表有没有数据。右关联（Right Join）相反，以右边的表为准。而内关联则是两张表都要有数据才能查询到。外关联则是只要任意一张表有数据就能查询并显示。

不能用where操作 ；如果使用where的话如果右表不存在匹配的行，那么不会返回数据，所以用Left join。

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