多次使用 `students = students.filter(...)`，这实际上会导致只有最后一个过滤器生效，因为每次赋值都会覆盖上一次的查询集

def student_list(request):
    students = Student.objects.all().order_by('id')
    search_query = request.GET.get('sel')  # 获取搜索表单的 'sel' 字段值
    if search_query:
        # //学号搜索
        students = students.filter(id__icontains=search_query)
        # //姓名搜索
        students = students.filter(name__icontains=search_query)
        # //性别搜索
        students = students.filter(sex__icontains=search_query)
        # //年龄搜索
        students = students.filter(age__icontains=search_query)
        # //班级搜索
        students = students.filter(st