First Normal Form

/*


Intro

This Kata is a short introduction to the topic of database normalization. It's just a simple example of a decently complex topic with some details omitted for simplicity; to all interested, I advise further study.

Background

Normalization is a process of removing data redundancy from the database. Reduction of duplicated data means a lesser probability of inconsistencies and greater flexibility. Normal Forms are steps between which different forms of data redundancy are removed from the table. For a higher normal form to be satisfied all lower normal forms must also be satisfied.

First Normal Form (1NF)

To achieve First Normal Form (1NF) every column in the table must be atomic i.e. it can't contain a collection of values.

Task

You will be provided with a single table restaurants from which you have to extract two tables conforming to the First Normal Form

The first table must contain restaurants without a column with collection of values - menu field should be removed.
The second table must contain a dish column and a restaurant_id column - matching dishes to their restaurants
Ordering is not important
The solution should leave the database with two tables (restaurants and dishes) which will be queried by the tests.
Input table

--------------------------------------------------------------------------------------------------------
| Table       | Column | Type   | Notes                                                                |
|-------------+--------+-------------------------------------------------------------------------------|
| restaurants | id     | int    |                                                                      |
|             | menu   | string | comma-separated list of unique dishes (dish name may contain spaces) |
|             | name   | string |                                                                      |    
|             | stars  | int    |                                                                      |
--------------------------------------------------------------------------------------------------------
Output tables

---------------------------------
| Table       | Column | Type   |
|-------------+--------+--------|
| restaurants | id     | int    |
|             | name   | string |    
|             | stars  | int    |
---------------------------------
-----------------------------------
| Table  | Column        | Type   |
|--------+---------------+--------|
| dishes | restaurant_id | int    |
|        | dish          | string |    
-----------------------------------



*/



CREATE TABLE dishes AS
       SELECT id AS restaurant_id,
              UNNEST(string_to_array(menu,','))AS dish
       FROM restaurants;
       
ALTER TABLE restaurants
DROP COLUMN menu;