175. Combine Two TablesLeetCode-LEFT JON 和RIGHT JOIN，两张表关联查询-java -sql入门

Posted 2020-10-02 皓浩浩皓

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


题目就是搜索两个关联表，分别从表中去关键字，但是记得特殊情况就是SELECT WHERE 的时候没有数据匹配就会返回空，但是你比如查询xx的住址可能是有人员信息么有地址
这个时候我们用LEFT JON 和RIGHT JOIN

什么意思呢，上语句查询结果你就懂了

 

#左连接就是左边查询的如果有数据右面如果没有匹配，那么右面的数据可以为空
#SELECT FirstName, LastName, City, State FROM Person left join Address on Person.PersonId=Address.PersonId;
#右连接就是右边查询的如果有数据左面如果没有匹配，那么右面的数据可以为空
SELECT FirstName, LastName, City, State FROM  Address RIGHT  join  Person on  Address.PersonId=Person.PersonId;