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Facebook SQL Interview Question Solving in PostgreSQL

In this video, we solve a popular Facebook SQL interview question from DataLemur that focuses on identifying records with no matching entries using set operations in PostgreSQL.

Problem Statement
You are given two tables containing data about Facebook pages and the likes associated with those pages.

Your task is to write a SQL query that returns the IDs of Facebook pages that have received zero likes.
The result should be sorted in ascending order by page ID.

Tables and Columns

Table: pages
Columns
page_id – integer
page_name – varchar

Example Input
page_id | page_name
20001 | SQL Solutions
20045 | Brain Exercises
20701 | Tips for Data Analysts

Table: page_likes
Columns
user_id – integer
page_id – integer
liked_date – datetime

Example Input
user_id | page_id | liked_date
111 | 20001 | 04/08/2022 00:00:00
121 | 20045 | 03/12/2022 00:00:00
156 | 20001 | 07/25/2022 00:00:00

Example Output
page_id
20701

Explanation
Page 20701 has no matching entries in the page_likes table, meaning it has received zero likes. Therefore, it is included in the output.

Note
The dataset used for evaluation may differ from this example. This sample is only for understanding the logic.

SQL Approach

We can solve this problem using the EXCEPT operator to find page IDs that exist in the pages table but not in the page_likes table.

SELECT page_id
FROM pages
EXCEPT
SELECT page_id
FROM page_likes
ORDER BY page_id;

This query returns all page IDs that do not appear in the page_likes table.

If you are preparing for Facebook or Data Science SQL interviews, this problem is a great way to practice set-based operations in PostgreSQL.

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