SELECT c.id_comment, c.id_parent, u.login, n.title_news, cat.category_name,
c.`comment`, c.date_time, c.is_active FROM comments c
    LEFT JOIN users u ON u.id = c.id_user
    LEFT JOIN news n ON n.id_news = c.id_news
    LEFT JOIN category cat ON cat.id_category = n.id_category
        ORDER BY c.date_time DESC LIMIT {$start},{$limit};

SELECT колонки
FROM таблицы
JOIN таблица
WHERE условия
GROUP BY колонки
HAVING условие
ORDER BY колонки

SELECT c.id_comment,
       c.id_parent,
       u.login,
       n.title_news,
       cat.category_name,
       c.`comment`,
       c.date_time,
       c.is_active
FROM comments c
LEFT JOIN users u ON u.id = c.id_user
LEFT JOIN news n ON n.id_news = c.id_news
LEFT JOIN category cat ON cat.id_category = n.id_category
WHERE c.is_active=1
ORDER BY c.date_time DESC
LIMIT {$start},{$limit};