某高校教学管理系统中的院系关系Department和学生关系Students的模式分别为：Department(院系号，院系名，负责人，办公电话)，学生关系Students(学号，姓名，身份证号，院系号，联系电话，家庭住址)。Department中的“院系号”唯一标识一个院系，Students中的“学号”能唯一标识一名学生，“家庭住址”可进一步分为邮编．省．市．街道。根据以上描述可知，关系Students的候选键为 （） ,“家庭住址”为 （） 。创建Students的SQL语句如下，请填补其中的空缺。CREATE TABLE Students(学号CHAR(8)PRIMARY KEY,姓名CHAR(16)，身份证号CHAR(18),院系号CHAR(4),联系电话CHAR(13),家庭住址CHAR(30)FOREIGN KEY （此空作答）。
- A.(联系电话)REFERENCES Department(办公电话)
- B.(院系号)REFERENCES Department(院系号)
- C.(院系号)REFERENCES Students(院系号)
- D.(负责人号)REFERENCES Students(学号)
正确答案及解析
正确答案
解析
在关系模型中，候选键又称候选码，是某个关系变量的一组属性所组成的集合，它需要同时满足下列两个条件：1、这个属性集合始终能够确保在关系中能唯一标识元组。2、在这个属性集合中找不出合适的真子集能够满足条件。在这里，学生的候选键有身份证号和学号。家庭住址是复合属性。院系号作为院系关系的主键，所以在学生关系中院系号是外键。
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