LeetCode：Combine Two Tables - 跨表查詢

一、題目名稱

Combine Two Tables（跨表查詢）

二、題目地址

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

三、題目內容

如今有兩張表Person和Address，它們的表結構以下：

表Person：

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

表Address：

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

寫一個SQL語句，查出每一個人的下列信息：FirstName, LastName, City, State

四、初始化數據庫腳本

在MySQL數據庫中創建一個名爲LEETCODE的數據庫，用MySQL命令行中的source命令執行下面腳本：

-- 執行腳本前必須創建名爲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', '關');
INSERT INTO Person (PersonId, FirstName, LastName)
  VALUES (2, '飛', '張');
INSERT INTO Person (PersonId, FirstName, LastName)
  VALUES (3, '超', '馬');
INSERT INTO Person (PersonId, FirstName, LastName)
  VALUES (4, '雲', '趙');
INSERT INTO Person (PersonId, FirstName, LastName)
  VALUES (5, '忠', '黃');
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', '河東');
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', '扶風');
INSERT INTO Address (AddressId, PersonId, City, State)
  VALUES (44, 4, '真定', '常山');
INSERT INTO Address (AddressId, PersonId, City, State)
  VALUES (55, 5, '南陽', '荊州');

五、解題SQL

解決本問題的SQL其實很是簡單，使用 LEFT JOIN 跨兩個表查詢一下就能夠了，SQL語句以下：

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

須要注意的是，不能使用下面這條SQL語句：

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

緣由是由於當Person表中有條目，而Address表中沒有時，條目就沒法被這條SQL查詢到了，而事實上在Person表中有的條目應該被所有查詢出來。

兩個SQL語句查詢結果的不一樣之處能夠從查詢結果中看出：

能夠看到，第一條SQL語句能夠查出魏延，第二條不能查出。

END