PRIMARY KEY( `id_athr`, `last_name`(25))

ADD PRIMARY KEY (`id_bk`, `name`(50))

ALTER TABLE `books` ADD FOREIGN KEY (`id_athr`) REFERENCES `authors`(`id_athr`)
ON UPDATE CASCADE
ON DELETE RESTRICT;

EXPLAIN SELECT 
    COUNT(*) as 'count', 
    `books`.`id_athr` AS 'id_auth',
    `authors`.`last_name` AS 'author_name',
    `authors`.`first_name` AS 'author_family'
FROM `books` LEFT JOIN `authors`
    ON `books`.`id_athr` = `authors`.`id_athr`
WHERE `authors`.`first_name` != 'Литагент' 
    AND `authors`.`first_name` != ''
GROUP BY `author_name`
ORDER BY `count` DESC;