Select post and concatenating all categories it belongs to
This should get all posts and a list of categories (If no categories, then 'None')

SELECT
    p.id, p.title, p.content,
    IFNULL(GROUP_CONCAT(p_c.category_id
    ORDER BY p_c.category_id ASC SEPARATOR ','),'None') category
FROM
    posts p
    LEFT JOIN posts_categories p_c ON p.id = p_c.post_id
    LEFT JOIN categories c ON p_c.category_id = c.id
WHERE
    p.id = 1
GROUP BY
    p.id
;    


Give it a Try !!!