When you have a lot of tables and data, it’s common to feel the need to “join” the tables based on some common information.

For example you have a products table and a orders table.

The orders table has the id of a product ordered in product_id (imagine an order can only be for 1 single product).

The products table has the product name in its name column.

Something like this:

CREATE TABLE orders (
  id SERIAL PRIMARY KEY, 
  buyer TEXT,
  product_id INTEGER
);

CREATE TABLE products (
  id SERIAL PRIMARY KEY,
  name TEXT
);

INSERT INTO orders (buyer, product_id) VALUES ('[email protected]', 1);
INSERT INTO orders (buyer, product_id) VALUES ('[email protected]', 2);

INSERT INTO products (name) VALUES ('milk');
INSERT INTO products (name) VALUES ('bread');

Now suppose you want to get the names of the products ordered by user with email [email protected], here’s a query you would use:

SELECT name FROM products INNER JOIN orders ON orders.product_id = products.id

Maybe you want the emails of people that bought milk:

SELECT orders.buyer FROM products INNER JOIN orders ON orders.product_id = products.id WHERE products.name = 'milk'

This is the simplest kind of join, called “inner join”.

Joins can be much more complex, but this is a start.

Lessons in this unit: