ORM作业参考答案：

查询平均成绩大于60分的同学的id和平均成绩；

 rows = Student.objects.annotate(avg=Avg("score__number")).filter(avg__gte=60).values("id","avg")
 for row in rows:
     print(row)

查询所有同学的id、姓名、选课的数、总成绩；

 rows = Student.objects.annotate(course_nums=Count("score__course"),total_score=Sum("score__number"))
 .values("id","name","course_nums","total_score")
 for row in rows:
     print(row)

查询姓“李”的老师的个数；

 teacher_nums = Teacher.objects.filter(name__startswith="李").count()
 print(teacher_nums)

查询没学过“黄老师”课的同学的id、姓名；

 rows = Student.objects.exclude(score__course__teacher__name="黄老师").values('id','name')
 for row in rows:
     print(row)

查询学过课程id为1和2的所有同学的id、姓名；

 rows = Student.objects.filter(score__course__in=[1,2]).distinct().values('id','name')
 for row in rows:
     print(row)

查询学过“黄老师”所教的所有课的同学的学号、姓名；

 rows = Student.objects.annotate(nums=Count("score__course",filter=Q(score__course__teacher__name='黄老师')))
 .filter(nums=Course.objects.filter(teacher__name='黄老师').count()).values('id','name')
 for row in rows:
     print(row)

查询所有课程成绩小于60分的同学的id和姓名；

 students = Student.objects.exclude(score__number__gt=60)
 for student in students:
     print(student)

查询没有学全所有课的同学的id、姓名；

 students = Student.objects.annotate(num=Count(F("score__course"))).filter(num__lt=Course.objects.count()).values('id','name')
 for student in students:
     print(student)

查询所有学生的姓名、平均分，并且按照平均分从高到低排序；

 students = Student.objects.annotate(avg=Avg("score__number")).order_by("-avg").values('name','avg')
 for student in students:
     print(student)

查询各科成绩的最高和最低分，以如下形式显示：课程ID，课程名称，最高分，最低分：

courses = Course.objects.annotate(min=Min("score__number"),max=Max("score__number")).values("id",'name','min','max')
for course in courses:
    print(course)

查询每门课程的平均成绩，按照平均成绩进行排序；

courses = Course.objects.annotate(avg=Avg("score__number")).order_by('avg').values('id','name','avg')
for course in courses:
    print(course)

统计总共有多少女生，多少男生；

rows = Student.objects.aggregate(male_num=Count("gender",filter=Q(gender=1)),female_num=Count("gender",filter=Q(gender=2)))
print(rows)

将“黄老师”的每一门课程都在原来的基础之上加5分；

rows = Score.objects.filter(course__teacher__name='黄老师').update(number=F("number")+5)
print(rows)

查询两门以上不及格的同学的id、姓名、以及不及格课程数；

students = Student.objects.annotate(bad_count=Count("score__number",filter=Q(score__number__lt=60))).filter(bad_count__gte=2).values('id','name','bad_count')
for student in students:
    print(student)

查询每门课的选课人数；

courses = Course.objects.annotate(student_nums=Count("score__student")).values('id','name','student_nums')
for course in courses:
    print(course)