Introduction to SQL Queries for Unread Messages and Last Message

When dealing with a large dataset of messages, it’s essential to have efficient queries that can fetch specific information. In this article, we’ll explore how to retrieve the number of unread messages and the last message sent between two users using SQL.

Background and Context

Let’s start by examining the provided table structure:

We want to write a SQL query that returns the number of unread messages and the last message for each user.

Writing the Query

To accomplish this task, we can use a combination of window functions, subqueries, and joins.

Step 1: Define the Problem and Requirements

Our goal is to retrieve the following information:

• The number of unread messages for each user who sends messages to another user or receives messages from that same user.
• The last message sent between two users.

We want to achieve this using a SQL query that is efficient, readable, and well-documented.

Step 2: Use a Common Table Expression (CTE) for Window Function

To calculate the number of unread messages and the last message, we can use a CTE with a window function. We’ll partition the data by from_user_id and sort it in descending order based on sent_date.

WITH ranked_messages AS (
    SELECT 
         from_user_id, to_user_id, body, sent_date,
         ROW_NUMBER() OVER (PARTITION BY from_user_id ORDER BY sent_date DESC) AS message_rank
    FROM messages
)

This CTE will assign a unique rank to each message based on the sent_date and the user who sent it.

Step 3: Calculate Unread Message Count

To calculate the number of unread messages, we can use a subquery that filters out read messages. We’ll count the number of rows where to_user_id = from_user_id AND is_read = false.

SELECT 
    ranked_messages.from_user_id, 
    ranked_messages.to_user_id, 
    ranked_messages.body, 
    ranked_messages.sent_date,
    (
        SELECT COUNT(*) FROM messages WHERE (to_user_id = ranked_messages.to_user_id OR from_user_id = ranked_messages.from_user_id) AND is_read = false
    ) AS unread_message_count
FROM ranked_messages

This subquery will return the total count of unread messages for each user.

Step 4: Join with Original Messages Table

To get the last message sent between two users, we can join our CTE with the original messages table. We’ll use an INNER JOIN to match rows where from_user_id = to_user_id.

SELECT 
    ranked_messages.from_user_id, 
    ranked_messages.to_user_id, 
    ranked_messages.body, 
    ranked_messages.sent_date,
    (
        SELECT COUNT(*) FROM messages WHERE (to_user_id = ranked_messages.to_user_id OR from_user_id = ranked_messages.from_user_id) AND is_read = false
    ) AS unread_message_count
FROM ranked_messages
INNER JOIN messages ON ranked_messages.from_user_id = messages.to_user_id
WHERE ranked_messages.message_rank = 1;

This join will return the last message sent between two users.

Step 5: Combine Results

Now that we have both the unread message count and the last message, we can combine them into a single result set.

SELECT 
    m.from_user_id, 
    m.to_user_id, 
    m.body, 
    m.sent_date,
    (
        SELECT COUNT(*) FROM messages WHERE (to_user_id = m.to_user_id OR from_user_id = m.from_user_id) AND is_read = false
    ) AS unread_message_count
FROM (
    SELECT 
         to_user_id, 
         body, 
         sent_date,
         ROW_NUMBER() OVER (PARTITION BY to_user_id ORDER BY sent_date DESC) AS message_rank
    FROM messages
    WHERE to_user_id = 31 OR from_user_id = 31
) m
INNER JOIN (
    SELECT 
         from_user_id, 
         to_user_id, 
         body, 
         sent_date,
         ROW_NUMBER() OVER (PARTITION BY from_user_id ORDER BY sent_date DESC) AS message_rank
    FROM messages
    WHERE to_user_id = 31 OR from_user_id = 31
) m2 ON m.to_user_id = m2.from_user_id
WHERE m.message_rank = 1;

This final query will return the number of unread messages and the last message sent between two users.

Conclusion

In this article, we explored how to write a SQL query that returns the number of unread messages and the last message sent between two users. We used a combination of window functions, subqueries, and joins to achieve this task. The final query will return a result set with both the unread message count and the last message for each user.

Example Use Cases

• Retrieving unread messages for a specific user
• Finding the last message sent between two users
• Calculating unread message counts for multiple users at once

Last modified on 2024-09-18