Postgresql Joins

PostgreSQL JOINS
A JOIN clause is used to combine rows from two or more tables, based on a related column between them.Let's look at a selection from the products table:

 product_id |  product_name  | category_id
------------+----------------+-------------
         33 | Geitost        |           4
         34 | Sasquatch Ale  |           1
         35 | Steeleye Stout |           1
         36 | Inlagd Sill    |           8

Then, look at a selection from the categories table:

category_id | category_name
-------------+----------------
           1 | Beverages
           2 | Condiments
           3 | Confections
           4 | Dairy Products

Notice that the category_id column in the products table refers to the category_id in the categories table. The relationship between the two tables above is the category_id column.

Then, we can create the following SQL statement (with a JOIN), that selects records that have matching values in both tables:

SELECT product_id, product_name, category_name
FROM products
INNER JOIN categories ON products.category_id = categories.category_id;

Result
 product_id |  product_name  | category_name
------------+----------------+----------------
         33 | Geitost        | Dairy Products
         34 | Sasquatch Ale  | Beverages
         35 | Steeleye Stout | Beverages
         36 | Inlagd Sill    | Seafood

Reference
https://www.w3schools.com/postgresql/postgresql_joins.php