Understanding Many-to-Many Relationships in Postgres

In this article, we will explore how to implement a many-to-many relationship between two tables using the question_response table in Postgres. We will also discuss how to insert multiple rows of data into this table.

What is a Many-To-Many Relationship?

A many-to-many relationship occurs when one entity can be related to multiple other entities, and vice versa. In our case, we have three tables: questions, responses, and question_response. A question can have multiple responses, and a response can belong to multiple questions.

Table Structure

Here is the table structure for the three tables:

questions.sql

CREATE TABLE questions(
    id BIGSERIAL PRIMARY KEY,
    question VARCHAR(255)
);

responses.sql

CREATE TABLE responses(
    id BIGSERIAL PRIMARY KEY,
    response VARCHAR(255)
);

question_response.sql

CREATE TABLE question_response(
    question_id bigint REFERENCES questions ON DELETE CASCADE,
    response_id bigint REFERENCES responses ON DELETE CASCADE,
    PRIMARY KEY (question_id, response_id)
);

Data Insertion

We want to insert the following data into the question_response table:

Using Cross Join to Insert Data

To insert this data, we can use a cross join between the responses and questions tables. A cross join returns the Cartesian product of both tables, which means every row in one table is combined with every row in the other table.

Here is an example query that does this:

INSERT INTO question_response(
    question_id,
    response_id
)
SELECT
    q.id AS question_id,
    r.id AS response_id
FROM
    responses r
CROSS JOIN
        questions q;

This query will insert the desired data into the question_response table.

How it Works

Let’s break down how this query works:

1. The SELECT statement is used to select the columns we want to insert into the question_response table.
2. We use the AS keyword to give aliases to the columns, so that we can refer to them by their new names (question_id and response_id).
3. In the FROM clause, we specify the tables we want to join: responses ( aliased as r) and questions (aliased as q).
4. We use a cross join operator (CROSS JOIN) to combine the rows of both tables.
5. The resulting table has every row from responses combined with every row from questions, which gives us the desired many-to-many relationship.

Example Use Case

Here is an example of how we can use this query in practice:

Suppose we want to analyze the responses to a particular question. We can join the question_response table with both the responses and questions tables using the corresponding IDs:

SELECT
    q.question,
    r.response,
    COUNT(*) AS num_responses
FROM
    question_response qr
JOIN
        questions q ON qr.question_id = q.id
JOIN
        responses r ON qr.response_id = r.id
WHERE
    qr.question_id = 1
GROUP BY
    q.question, r.response;

This query will return the response values for a particular question, along with the number of times each response was selected.

Conclusion

In this article, we explored how to implement a many-to-many relationship between two tables using the question_response table in Postgres. We discussed how to insert multiple rows of data into this table and provided an example query that uses cross join to achieve this.

Last modified on 2024-04-02