Сложные запросы

//ВЫбрать количество предметов у каждой группы
SELECT [group].group_code, COUNT (subject_code) FROM [group] JOIN teaching_load
ON [group].group_code=teaching_load.group_code GROUP BY [group].group_code

//Количество часов у каждого преподавателя
SELECT teachers.last_name, teachers.first_name, teachers.middle_name, SUM (teaching_load.hours) AS hours_count
FROM teachers LEFT JOIN teaching_load
ON teachers.teacher_code=teaching_load.teacher_code
GROUP BY teachers.last_name, teachers.first_name, teachers.middle_name
ORDER BY hours_count DESC

//У которых не назначены часы
SELECT teachers.last_name, teachers.first_name, teachers.middle_name
FROM teachers WHERE NOT EXISTS (SELECT * from teaching_load WHERE teaching_load.teacher_code=teachers.teacher_code)