175 - Combine Two Tables

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The problem

This is a database related problem from Leetcode. You can find the problem here.

Description

Table: Person

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| personId    | int     |
| lastName    | varchar |
| firstName   | varchar |
+-------------+---------+
personId is the primary key (column with unique values) for this table.
This table contains information about the ID of some persons and their first and last names.

Table: Address

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| addressId   | int     |
| personId    | int     |
| city        | varchar |
| state       | varchar |
+-------------+---------+
addressId is the primary key (column with unique values) for this table.
Each row of this table contains information about the city and state of one person with ID = PersonId.

Write a solution to report the first name, last name, city, and state of each person in the Person table. If the address of a personId is not present in the Address table, report null instead.

Return the result table in any order.

The result format is in the following example.

SQL Schema

Create table If Not Exists Person (personId int, firstName varchar(255), lastName varchar(255))
Create table If Not Exists Address (addressId int, personId int, city varchar(255), state varchar(255))
Truncate table Person
insert into Person (personId, lastName, firstName) values ('1', 'Wang', 'Allen')
insert into Person (personId, lastName, firstName) values ('2', 'Alice', 'Bob')
Truncate table Address
insert into Address (addressId, personId, city, state) values ('1', '2', 'New York City', 'New York')
insert into Address (addressId, personId, city, state) values ('2', '3', 'Leetcode', 'California')

Examples

Input: 
Person table:
+----------+----------+-----------+
| personId | lastName | firstName |
+----------+----------+-----------+
| 1        | Wang     | Allen     |
| 2        | Alice    | Bob       |
+----------+----------+-----------+

Address table:
+-----------+----------+---------------+------------+
| addressId | personId | city          | state      |
+-----------+----------+---------------+------------+
| 1         | 2        | New York City | New York   |
| 2         | 3        | Leetcode      | California |
+-----------+----------+---------------+------------+

Output: 
+-----------+----------+---------------+----------+
| firstName | lastName | city          | state    |
+-----------+----------+---------------+----------+
| Allen     | Wang     | Null          | Null     |
| Bob       | Alice    | New York City | New York |
+-----------+----------+---------------+----------+
Explanation: 
There is no address in the address table for the personId = 1 so we return null in their city and state. addressId = 1 contains information about the address of personId = 2.

Solving

This is a simple problem that can be solved using a LEFT JOIN query.

-- LEFT JOIN with ON condition
SELECT p.firstName, p.lastName, a.city, a.state
FROM Person p
LEFT JOIN Address a
ON p.personId = a.personId;

-- LEFT JOIN with USING condition
SELECT p.firstName, p.lastName, a.city, a.state
FROM Person p
LEFT JOIN Address a
USING (personId);