家庭作业 | AppMaster

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完善后台的数据模型和业务流程，使城市的人口信息以千人为单位计算，并以 "XX，XXX "的格式显示（同时，数据库中会存储精确到一人的总人数）。
如果没有指定一个城市属于哪个国家的信息，则禁止创建该城市。
显示关于世界的摘要信息。城市总数，人口总数，最大和最小的城市，成立日期未知的城市数量。
添加一个按钮，删除所选国家的所有城市。