ar_snippet_sql_2.sql

SELECT `boats`.*
FROM `boats`
  INNER JOIN `boat_classifications`
    ON `boat_classifications`.`boat_id` = `boats`.`id`
  INNER JOIN `classifications`
    ON `classifications`.`id` = `boat_classifications`.`classification_id`
GROUP BY `boats`.`id`
HAVING count(classifications.id) = 3