Chain of Linked Tables

Now suppose users live in certain cities, and these cities are located in different countries. In this case, for storage we will need three tables: users will be linked to cities, and cities - to countries. At the same time, we won't need a field linking users to countries - because users will already be linked to countries through the link between cities and countries.

Let's look at our tables. The table with countries:

countries
id	name
1	country1
2	country2

The table with cities:

cities
id	name	country_id
1	city1	1
2	city2	1
3	city3	2

The table with users will remain unchanged:

users
id	name	city_id
1	user1	1
2	user2	1
3	user3	2
4	user4	1
5	user5	3
6	user6	2

Queries

In order to get users along with their cities and countries, we will have to make two joins: the first will join cities to users, and the second - countries to cities:

SELECT 
	users.name,
	cities.name as city_name,
	countries.name as country_name
FROM 
	users
LEFT JOIN cities ON cities.id=users.city_id
LEFT JOIN countries ON countries.id=cities.country_id

Practical Tasks

Suppose products belong to a certain subcategory, and subcategories belong to a certain category. Describe the storage structure.

Write a query that will fetch products, along with their subcategories and categories.

Write a query that will fetch subcategories along with their categories.