SQL Query to Select Mutual Rows: A Deep Dive

In this article, we’ll explore a common problem in data analysis: selecting rows that have mutual responses between two IDs. We’ll delve into the world of SQL queries, focusing on PostgreSQL as an example database management system.

Background and Problem Statement

Imagine you’re working with a dataset that contains source and destination IDs along with messages exchanged between them. You want to identify rows where there’s a mutual response for a given ID (e.g., id 1). In other words, you want to find rows where both the source ID (source_id) and destination ID (destination_id) are involved in the exchange.

Let’s consider an example table named messages with the following structure:

# Table: messages

| Column Name | Data Type |
|-------------|-----------|
| id          | integer   |
| source_id   | integer   |
| destination_id| integer   |
| message     | text       |

The data would look something like this:

We’re interested in selecting rows where both source_id and destination_id are involved in the exchange, but only if they have a mutual response (i.e., one message is sent by source_id to destination_id, and another message is sent by destination_id to source_id). We’ll explore different SQL queries to achieve this.

Solution Overview

We’ll use PostgreSQL’s exists clause to find rows that meet the criteria. This involves two subqueries: one to identify potential pairs of source-destination relationships, and another to verify whether a mutual response exists between those relationships.

Subquery 1: Identifying Potential Pairs

The first step is to select all possible combinations of source_id and destination_id that can form a pair. We’ll use the following subquery:

SELECT source_id, destination_id FROM messages;

This query returns two columns, source_id and destination_id, which represent potential pairs.

Subquery 2: Verifying Mutual Responses

Next, we need to verify whether a mutual response exists between each pair. We’ll use another subquery:

SELECT m1.source_id AS source_id, m1.destination_id AS destination_id FROM messages m1
JOIN messages m2 ON m1.source_id = m2.destination_id AND m1.destination_id = m2.source_id;

This query finds pairs where both source_id and destination_id are involved in the exchange. However, this alone isn’t enough to ensure a mutual response.

Combining Subqueries with EXISTS

To address this limitation, we’ll modify our approach. We’ll use the exists clause to verify whether a mutual response exists between each pair:

SELECT m.*
FROM messages m
WHERE EXISTS (
  SELECT 1 FROM messages m2
  WHERE m2.source_id = m.destination_id AND
        m2.destination_id = m.source_id
);

This query returns rows where both source_id and destination_id are involved in a mutual response.

Including ID Filters

If you want to include only IDs that meet the criteria (e.g., id 1), you can add an additional filter:

SELECT m.*
FROM messages m
WHERE id = 1 AND EXISTS (
  SELECT 1 FROM messages m2
  WHERE m2.source_id = m.destination_id AND
        m2.destination_id = m.source_id
);

This query returns only rows with id equal to 1, along with the rows that meet the mutual response criteria.

Conclusion

In this article, we explored the task of selecting rows with mutual responses between two IDs. We used PostgreSQL’s exists clause to verify whether a mutual response exists between each pair. By combining subqueries and filters, we developed a robust SQL query to address this common problem in data analysis.

Last modified on 2023-12-23