LeetCode：Combine Two Tables - 跨表查询

1、题目名称

Combine Two Tables（跨表查询）

2、题目地址

https://leetcode.com/problems/combine-two-tables/

3、题目内容

现在有两张表Person和Address，它们的表结构如下：

表Person：

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| PersonId    | int     |
| FirstName   | varchar |
| LastName    | varchar |
+-------------+---------+

表Address：

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| AddressId   | int     |
| PersonId    | int     |
| City        | varchar |
| State       | varchar |
+-------------+---------+

写一个SQL语句，查出每个人的下列信息：FirstName, LastName, City, State

4、初始化数据库脚本

在MySQL数据库中建立一个名为LEETCODE的数据库，用MySQL命令行中的source命令执行下面脚本：

-- 执行脚本前必须建立名为LEETCODE的DATABASE
USE LEETCODE;

DROP TABLE IF EXISTS Person;
CREATE TABLE Person (
  PersonId INT NOT NULL PRIMARY KEY,
  FirstName VARCHAR(50),
  LastName VARCHAR(50)
);

INSERT INTO Person (PersonId, FirstName, LastName)
  VALUES (1, '羽', '关');
INSERT INTO Person (PersonId, FirstName, LastName)
  VALUES (2, '飞', '张');
INSERT INTO Person (PersonId, FirstName, LastName)
  VALUES (3, '超', '马');
INSERT INTO Person (PersonId, FirstName, LastName)
  VALUES (4, '云', '赵');
INSERT INTO Person (PersonId, FirstName, LastName)
  VALUES (5, '忠', '黄');
INSERT INTO Person (PersonId, FirstName, LastName)
  VALUES (6, '延', '魏');

DROP TABLE IF EXISTS Address;
CREATE TABLE Address (
  AddressId INT NOT NULL PRIMARY KEY,
  PersonId INT,
  City VARCHAR(50),
  State VARCHAR(50)
);

INSERT INTO Address (AddressId, PersonId, City, State)
  VALUES (11, 1, '解良', '河东');
INSERT INTO Address (AddressId, PersonId, City, State)
  VALUES (22, 2, '涿郡', '幽州');
INSERT INTO Address (AddressId, PersonId, City, State)
  VALUES (33, 3, '茂陵', '扶风');
INSERT INTO Address (AddressId, PersonId, City, State)
  VALUES (44, 4, '真定', '常山');
INSERT INTO Address (AddressId, PersonId, City, State)
  VALUES (55, 5, '南阳', '荆州');

5、解题SQL

解决本问题的SQL其实非常简单，使用 LEFT JOIN 跨两个表查询一下就可以了，SQL语句如下：

SELECT A.FIRSTNAME, A.LASTNAME, B.CITY, B.STATE 
FROM Person AS A 
LEFT JOIN Address AS B 
ON A.PERSONID = B.PERSONID;

需要注意的是，不能使用下面这条SQL语句：

SELECT A.FirstName, A.LastName, B.City, B.State 
FROM Person AS A, Address AS B 
WHERE A.PersonId = B.PersonId;

原因是因为当Person表中有条目，而Address表中没有时，条目就无法被这条SQL查询到了，而事实上在Person表中有的条目应该被全部查询出来。

两个SQL语句查询结果的不同之处可以从查询结果中看出：

可以看到，第一条SQL语句可以查出魏延，第二条不能查出。

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转载于:https://my.oschina.net/Tsybius2014/blog/492966