5. Find Popular Posts (Easy)

Grokking SQL for Tech Interviews

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5. Find Popular Posts (Easy)

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Problem Statement

Table: Posts
Each row in this table represents a post made by a user in a social media platform. It contains the unique post ID, the user who posted it, and the number of likes received by that post.

+-------------+---------+
| Column Name | Type    | 
+-------------+---------+
| post_id     | int     | 
| user_id     | int     |
| post_likes  | int     |
+-------------+---------+

(post_id, user_id) is a unique primary key in the above table.
post_id is the primary key for this table.
user_id refers to the ID of the user who has liked the post.

Develop a solution to find the number of likes for each post.

Return the result table ordered by post_id in ascending order.

Example

Input:

Posts table:
+---------+---------+
| post_id | user_id |
+---------+---------+
| 1       | 2       | 
| 2       | 1       |
| 1       | 3       |            
| 3       | 1       |
| 1       | 4       |            
| 4       | 2       |           
| 2       | 2       |
| 4       | 3       |             
+---------+---------+

Output:

+---------+-------------+
| post_id | post_likes  |
+---------+-------------+
| 1       | 3           |
| 2       | 2           |
| 3       | 1           |
| 4       | 2           |
+---------+-------------+

Try It Yourself

MYSQL

. . . .

Solution

To determine the number of likes each post has received, we analyze the Posts table. Each row in the Posts table represents a like made by a user (user_id) on a specific post (post_id). By aggregating this data, we can count how many users have liked each post, effectively determining the total number of likes per post.

Here are the steps for the solution.

Aggregate Likes per Post:
- Count the number of user_ids associated with each post_id to determine the total likes per post.
Order the Results:
- Present the final count of likes in ascending order of post_id for clarity and organization.

SQL Query

SELECT 
    post_id, 
    COUNT(user_id) AS post_likes
FROM 
    Posts
GROUP BY 
    post_id
ORDER BY 
    post_id ASC;

Step-by-Step Approach

Step 1: Select Relevant Columns

Identify the columns necessary for calculating the number of likes per post.

SQL Snippet:

SELECT 
    post_id, 
    COUNT(user_id) AS post_likes

Explanation:

SELECT post_id,
- *Retrieve the post_id to group likes per post.
COUNT(user_id) AS post_likes
- COUNT(user_id) counts the number of non-NULL user_id entries for each post_id.
- AS post_likes aliases the count as post_likes for clarity in the output.

Step 2: Aggregate Data Using GROUP BY

Group the data by post_id to perform aggregation on a per-post basis.

SQL Snippet:

GROUP BY 
    post_id

Explanation:

Groups all rows that have the same post_id into a single group.
The COUNT(user_id) function then operates within each group to count the number of likes.

Intermediate Output After Step 3:

+---------+-------------+
| post_id | post_likes  |
+---------+-------------+
| 1       | 3           |
| 2       | 2           |
| 3       | 1           |
| 4       | 2           |
+---------+-------------+

Step 3: Order the Results

Sort the final results by post_id in ascending order for organized presentation.

SQL Snippet:

ORDER BY 
    post_id ASC;

Explanation:

ORDER BY post_id ASC;
- ORDER BY post_id specifies the column to sort by.
- ASC denotes ascending order (from smallest to largest).

Final Output:

+---------+-------------+
| post_id | post_likes  |
+---------+-------------+
| 1       | 3           |
| 2       | 2           |
| 3       | 1           |
| 4       | 2           |
+---------+-------------+

.....

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