Could you provide a SQL query to find the set of unique communicators from a table named messages containing the fields id, sender_id, receiver_id, and message?

Question Analysis

The question asks for a SQL query to identify all unique communicators from a table named messages. The table contains the following fields:

• id: An identifier for each message.
• sender_id: The ID of the user who sent the message.
• receiver_id: The ID of the user who received the message.
• message: The content of the message.

To solve this problem, we need to extract unique user IDs from both the sender_id and receiver_id columns, as both senders and receivers are considered communicators. The result should be a distinct list of user IDs who either sent or received a message.

Answer

To find the set of unique communicators from the messages table, you can use the following SQL query:

SELECT DISTINCT user_id 
FROM (
    SELECT sender_id AS user_id FROM messages
    UNION
    SELECT receiver_id AS user_id FROM messages
) AS communicators;

Explanation:

• Subquery: The subquery uses UNION to combine sender_id and receiver_id into a single column named user_id. The UNION operator inherently removes duplicates, which helps in identifying unique user IDs across both columns.

• Outer Query: The outer query then selects distinct user_id values from the result of the subquery, ensuring that each communicator is listed only once.

This query effectively compiles a list of unique user IDs that have either sent or received a message, fulfilling the requirement of the question.