Never Ever

SELECT AuthorName, COUNT(authorId ) as authorId
 FROM `book` LEFT JOIN autor ON authorId = id
 GROUP BY AuthorName 
 HAVING authorId >= 3

SELECT MAX(`count`), product_id FROM `sales` WHERE product_id='.$productID.

потому что тут только одна колонка должна быть, выкинь product_id

SET @del := (SELECT CONCAT('DELETE FROM ', GROUP_CONCAT(TABLE_NAME SEPARATOR ' WHERE id=0; DELETE FROM ') , ';' )
  AS statement FROM information_schema.TABLES WHERE TABLE_NAME LIKE '%table_%' );
PREPARE stmt FROM @del;
EXECUTE stmt;

SELECT * FROM Table ORDER BY score DESC LIMIT 10