Sub query

Databases
1

i have 2 tables called products and categories, i have categories_id is a foreign key in the table products, but when i did that query it calls NULL.
i want to use the value of the foreign key to make sub query to select the category name from the table categories and return the name of the category with the products query, here is the code .

select products_name, products_image, 
products_art_number, products_color, (select categories_name 
from categories where categories_id='categories_id'), sub_categories_id
 from products where products_id = '$this->id'
2

thanx

3

thanx Blue it works, but when i tried this out with 2 tables it returns error

SELECT
    p.products_name,
    p.products_image,
    p.products_art_number,
    p.products_color,
    c.categories_name
    s.sub_categories
FROM
    products p
LEFT JOIN
    categories c,
    sub_categories s,
ON
    c.categories_id = p.categories_id,
    s.sub_categories_id = p.sub_categories_id
WHERE
    p.products_id = '$this->id'
4 
SELECT p.products_name
     , p.products_image
     , p.products_art_number
     , p.products_color
     , c.categories_name
     , s.sub_categories
  FROM products p
INNER
  JOIN categories c
    ON c.categories_id = p.categories_id
INNER
  JOIN sub_categories s
    ON s.sub_categories_id = p.sub_categories_id
 WHERE p.products_id = '$this->id'
5 
SELECT
    p.products_name, 
    p.products_image, 
    p.products_art_number, 
    p.products_color,
    c.categories_name
FROM 
    products p
LEFT JOIN 
    categories c 
ON
    c.categories_id = p.categories_id
WHERE 
    p.products_id = '$this->id'