I would like to start a simple wikepedia database system using postgreSQL with the following,

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  username TEXT UNIQUE NOT NULL,
  email TEXT UNIQUE NOT NULL,
  password TEXT NOT NULL
);

CREATE TABLE categories (
  id SERIAL PRIMARY KEY,
  name TEXT UNIQUE NOT NULL
);

CREATE TABLE articles (
  id SERIAL PRIMARY KEY,
  title TEXT NOT NULL,
  content TEXT NOT NULL,
  created_at TIMESTAMP NOT NULL DEFAULT now(),
  user_id INTEGER REFERENCES users(id) ON DELETE CASCADE
);

CREATE TABLE categories_articles (
  category_id INTEGER REFERENCES categories(id) ON DELETE CASCADE,
  article_id INTEGER REFERENCES articles(id) ON DELETE CASCADE,
  PRIMARY KEY (category_id, article_id)
);

          +----------+        +------------+
          | users    |        | articles   |
          +----------+        +------------+
          | id       |------->| id         |
          | username |        | title      |
          | email    |        | content    |
          | password |        | created_at |
          +----------+        | user_id    |
                              +------------+

          +-----------+        +------------+
          | categories|        | articles   |
          +-----------+        +------------+
          | id        |------> | id         |
          | name      |        | title      |
          +-----------+        | content    |
                               | created_at |
                               | user_id    |
                               +------------+

          +---------------------+  
          | categories_articles |    
          +---------------------+  
          | category_id         |
          | article_id          |
          +---------------------+       

1. To use the categories_articles table in SQL, you can use SQL statements to insert, update, and delete rows in the table. Here are a few examples:
   • To insert a row into the categories_articles table linking an article with ID 1 to a category with ID 2:

INSERT INTO categories_articles (category_id, article_id) VALUES (2, 1);

2. To update the category of an article in the categories_articles table, assuming the article has ID 1 and you want to change its category to the one with ID 3:

UPDATE categories_articles SET category_id = 3 WHERE article_id = 1;

3. To delete a row from the categories_articles table linking an article with ID 1 to a category with ID 2:

DELETE FROM categories_articles WHERE category_id = 2 AND article_id = 1;

Note that when you insert or update a row in the categories_articles table, you need to make sure that the category_id and article_id values you are inserting or updating correspond to valid categories and articles that exist in the categories and articles tables.

4. You may want to use SQL joins to select data from these tables, as well as the categories_articles table, based on certain criteria. For example, to select all articles that belong to a certain category, you could use a query like this:

SELECT articles.title, articles.content
FROM articles
JOIN categories_articles ON articles.id = categories_articles.article_id
WHERE categories_articles.category_id = 2;

This query selects the title and content columns from the articles table, and joins it with the categories_articles table on the id and article_id columns. The WHERE clause filters the results to only include articles that belong to the category with ID 2.