175. Combine Two Tables

 

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

 

LeetCode还出了是来到数据库的题,来那么也来做做吧,这道题是第一道,相对来说比较简单,是一道两表联合查找的问题,我们需要用到Join操作,关于一些Join操作可以看我之前的博客SQL Left Join, Right Join, Inner Join, and Natural Join 各种Join小结,最直接的方法就是用Left Join来做,根据PersonId这项来把两个表联合起来:

 

解法一:

SELECT Person.FirstName, Person.LastName, Address.City, Address.State FROM Person LEFT JOIN Address ON Person.PersonId = Address.PersonId;

 

在使用Left Join时,我们也可以使用关键Using来声明我们相用哪个列名来进行联合:

 

解法二:

SELECT Person.FirstName, Person.LastName, Address.City, Address.State FROM Person LEFT JOIN Address USING(PersonId);

 

或者我们可以加上Natural关键字,这样我们就不用声明具体的列,MySQL可以自行搜索相同的列:

 

解法三:

SELECT Person.FirstName, Person.LastName, Address.City, Address.State FROM Person NATURAL LEFT JOIN Address;

 

参考资料:

https://leetcode.com/discuss/21216/its-a-simple-question-of-left-join-my-solution-attached

https://leetcode.com/discuss/53001/comparative-solution-between-left-using-natural-left-join

 

LeetCode All in One 题目讲解汇总(持续更新中…)

 


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