MySQL组合两个表

SQL架构

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');

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

题解

方法：使用 outer join

算法

因为表 Address 中的 personId 是表 Person 的外关键字，所以我们可以连接这两个表来获取一个人的地址信息。

考虑到可能不是每个人都有地址信息，我们应该使用 outer join 而不是默认的 inner join。

方式1

mysql> select firstName,LastName,City,State from Person p left join Address a on p.PersonId = a.PersonId;
+-----------+----------+------+-------+
| firstName | LastName | City | State |
+-----------+----------+------+-------+
| Allen     | Wang     | NULL | NULL  |
+-----------+----------+------+-------+
1 row in set (0.00 sec)

注意：如果没有某个人的地址信息，使用 where 子句过滤记录将失败，因为它不会显示姓名信息。

方式2

select 
    FirstName, 
    LastName, 
    (select City from Address a where a.PersonId=p.PersonId) as City, 
    (select State from Address a where a.PersonId=p.PersonId) as State  
from Person p;

+-----------+----------+------+-------+
| firstName | LastName | City | State |
+-----------+----------+------+-------+
| Allen     | Wang     | NULL | NULL  |
+-----------+----------+------+-------+
1 row in set (0.00 sec)