- A.若 X→Y,X→Z,则 X→YZ 为 F 所蕴涵
- B.若 X→Y,WY→Z,则 XW→Z 为 F 所蕴涵
- C.若 X→Y,Y→Z 为 F 所蕴涵,则 X→Z 为 F 所蕴涵
- D.若Ⅹ→Y 为 F 所蕴涵,且 Z?U,则 XZ→YZ 为 F 所蕴涵
正确答案及解析
正确答案
解析
从已知的一些函数依赖，可以推导出另外一些函数依赖，这就需要一系列推理规则。函数依赖的推理规则最早出现在1974年W.W.Armstrong 的论文里，这些规则常被称作"Armstrong 公理".合并规则：若X→Y，X→Z同时在R上成立，则X→YZ在R上也成立。分解规则：若X→W在R上成立，且属性集Z包含于W，则X→Z在R上也成立。伪传递规则：若X→Y在R上成立，且WY→Z，则XW→Z。
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