Listing Common Data from Two Tables Based on Foreign Key in PostgreSQL

A guide on how to list common data from two tables in PostgreSQL based on a foreign key, utilizing the JOIN statement for efficient querying.

In this article, we will explore how to use PostgreSQL to list common data from two tables through a foreign key. We will utilize the JOIN statement to connect the tables and retrieve the data.

SQL code

-- Assuming we have two tables: 'users' and 'orders'
-- 'users' has id as the primary key and 'orders' has user_id linked to users' id

-- Create the 'users' table
CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100) NOT NULL
);

-- Create the 'orders' table
CREATE TABLE orders (
    order_id SERIAL PRIMARY KEY,
    user_id INT REFERENCES users(id),
    item VARCHAR(100) NOT NULL
);

-- Insert data into the 'users' table
INSERT INTO users (name) VALUES ('Alice'), ('Bob'), ('Charlie');

-- Insert data into the 'orders' table
INSERT INTO orders (user_id, item) VALUES (1, 'Laptop'), (2, 'Phone'), (1, 'Tablet');

-- Query to list common data from both tables
SELECT u.id, u.name, o.order_id, o.item
FROM users u
JOIN orders o ON u.id = o.user_id;

Detailed explanation

• Create the users table: Use the CREATE TABLE statement to create the users table with fields id (primary key) and name.
• Create the orders table: Create the orders table with fields order_id (primary key), user_id (linked to users id), and item.
• Insert data into the users table: Use the INSERT INTO statement to add data to the users table.
• Insert data into the orders table: Add data to the orders table, where user_id links to the id of the users table.
• JOIN query: Use the SELECT statement with JOIN to connect the users and orders tables, retrieving the fields id, name, order_id, and item from both tables.

System Requirements:

• PostgreSQL 9.0 or later

How to install PostgreSQL:

You can install PostgreSQL from the official PostgreSQL website or use management tools like pgAdmin.

Tips:

• Use JOIN to efficiently connect tables and retrieve data in PostgreSQL.
• Ensure that the linking fields between tables are defined with foreign keys to maintain data integrity.

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