Combine Two Tables SQL Challenge - PostgreSQL Solution

1. Introduction

This blog post demonstrates how to merge data from two tables in a PostgreSQL database. We'll be using the Person and Address tables as examples to illustrate this process.


Our aim is to combine data from the Person and Address tables to display each person's first name, last name, city, and state. When a person's address isn't available in the Address table, we need to show null values for the city and state.

Database Table:

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

3. PostgreSQL Solution

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


firstName lastName city state
Allen Wang Null Null
Bob Alice New York City New York


1. A LEFT JOIN combines the Person and Address tables.

2. We select firstName and lastName from Person.

3. The city and state are taken from Address.

4. If there's no matching personId in Address, null is shown for city and state.