Django-查询相关练习

一：准备表结构

from django.db import models

# Create your models here.

class Student(models.Model):
    sid=models.AutoField(primary_key=True)
    sname=models.CharField(max_length=32)
    gender=models.CharField(max_length=32)
    clas=models.ForeignKey(to="Class",on_delete=models.CASCADE)
    def __str__(self):
        return self.sname

class Class(models.Model):
    cid=models.AutoField(primary_key=True)
    caption=models.CharField(max_length=32)
    grade=models.ForeignKey(to="Class_grade",on_delete=models.CASCADE)
    teacher=models.ManyToManyField(to='Teacher')
    def __str__(self):
        return self.caption

class Class_grade(models.Model):
    gid=models.AutoField(primary_key=True)
    gname=models.CharField(max_length=32)
    def __str__(self):
        return self.gname


class Course(models.Model):
    cid=models.AutoField(primary_key=True)
    cname=models.CharField(max_length=32)
    teacher=models.ForeignKey(to='Teacher',on_delete=models.CASCADE)
    def __str__(self):
        return self.cname

class Teacher(models.Model):
    tid=models.AutoField(primary_key=True)
    tname=models.CharField(max_length=32)
    def __str__(self):
        return self.tname
class Score(models.Model):
    sid=models.AutoField(primary_key=True)
    student=models.ForeignKey(to='Student',on_delete=models.CASCADE)
    course=models.ForeignKey(to='Course',on_delete=models.CASCADE)
    score=models.IntegerField()
    def __str__(self):
        return self.score

 

二：习题

# # 1 查询学生总人数；
    # ret1=Student.objects.count()
    # print(ret1)
    # # 2 查询“生物”课程和“物理”课程成绩都及格的学生id和姓名；
    # ret2=Score.objects.filter(course__cname__in=["生物","物理"],score__gte=60).values("student__sname","student__pk")
    #
    # # 3 查询每个年级的班级数，取出班级数最多的前三个年级名称；
    # ret3=Grade.objects.annotate(c=Count("klass")).order_by("-c")[:3]
    # print(ret3)
    #
    # # 4 查询平均成绩最高的学生的id和姓名以及平均成绩；
    # ret=Score.objects.values("student").annotate(avg_score=Avg("score")).order_by("-avg_score").values("student__sname","student__pk","avg_score")[0]
    # print(ret)
    #
    # # 5 查询每个年级的学生人数；
    # ret=Grade.objects.annotate(c=Count("klass__student")).values("gname","c")
    # print(ret)
    #
    # # 6 查询每位学生的学号，姓名,平均成绩；
    #
    # ret=Student.objects.annotate(avg_score=Avg("score__score")).values("sname","pk","avg_score")
    # print(ret)

    # 7 查询学生编号为“2”的学生的姓名、该学生成绩最高的课程名及分数；
    # ret=Score.objects.values("student").annotate(max_score=Max("score")).values("student__sname","max_score","course__cname")
    # ret = Score.objects.filter(student=2).order_by("-score").values("score", "course__cname", "student__sname")[0]
    # print(ret)

    # # 8 查询每一个姓“李”的老师所带班级数；
    # ret=Teacher.objects.filter(tname__startswith="李").annotate(c=Count("classes")).values("tname","c")

    # # 9 查询班级数小于5的年级id和年级名；
    # ret=Grade.objects.annotate(c=Count("klass")).filter(c__lt=5).values("pk","gname")

    # # 10 查询教过课程超过2门的老师的id和姓名；
    # ret=Teacher.objects.annotate(c=Count("course")).filter(c__gt=2).values("pk","tname")
    #
    # # 11 查询学过编号“1”课程和编号“2”课程的同学的学号、姓名；
    # ret=Student.objects.filter(score__course__cid__in=[1,2]).values("pk","sname")
    #
    # # 12 查询所带班级数最多的老师id和姓名；
    # ret=Teacher.objects.annotate(c=Count("classes")).order_by("-c").values("pk","tname")[0]
    #
    # # 13 查询有课程成绩小于60分的同学的学号、姓名；
    # ret=Score.objects.filter(score__lt=60).values("student__sname","student__pk").distinct()
    #
    # # 14 查询男生、女生的人数，按倒序排列；
    # ret=Student.objects.values("gender").annotate(c=Count(1)).order_by("c").values("gender","c")
    #
    # # 15 查询各个课程及相应的选修人数；
    # ret=Score.objects.values("course").annotate(c=Count("student")).values("course__cname","c")
    #
    # # 16 查询同时选修了物理课和生物课的学生id和姓名；
    # ret=Student.objects.filter(score__course__cname__in=["物理","生物"]).values("pk","sname")
    #
    # # 17 检索“3”课程分数小于60，按分数降序排列的同学学号；
    # ret=Score.objects.filter(course__cid=3,score__lt=60).order_by("score").values("student__sname","student__pk")
    #
    # # 18 查询每门课程的平均成绩，结果按平均成绩升序排列，平均成绩相同时，按课程号降序排列；
    # ret=Score.objects.values("course").annotate(avg_score=Avg("score")).order_by("avg_score","course")
    #
    # # 19 查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分；
    # ret=Course.objects.annotate(max_score=Max("score__score"),min_score=Min("score__score")).values("cname","max_score","min_score")