在线名词解释大全 - 数据库系统 - 详细

假设A1,…,An是E1相关的属性,B1,B2,…,Bm是E2相关的属性。下列哪个变换是正确的_______。

A
https://exam-online.oss-cn-beijing.aliyuncs.com/2106a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106d.png/>,key:D}]、[{"value":" class="import-img" ","key":"A"},{"value":" class="import-img" ","key":"B"},{"value":" class="import-img" ","key":"C"},{"value":" class="import-img" ","key":"D"}]
B
https://exam-online.oss-cn-beijing.aliyuncs.com/2106a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106d.png/>,key:D}]、[{"value":" class="import-img" ","key":"A"},{"value":" class="import-img" ","key":"B"},{"value":" class="import-img" ","key":"C"},{"value":" class="import-img" ","key":"D"}]
C
https://exam-online.oss-cn-beijing.aliyuncs.com/2106a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106d.png/>,key:D}]、[{"value":" class="import-img" ","key":"A"},{"value":" class="import-img" ","key":"B"},{"value":" class="import-img" ","key":"C"},{"value":" class="import-img" ","key":"D"}]
D
https://exam-online.oss-cn-beijing.aliyuncs.com/2106a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106d.png/>,key:D}]、[{"value":" class="import-img" ","key":"A"},{"value":" class="import-img" ","key":"B"},{"value":" class="import-img" ","key":"C"},{"value":" class="import-img" ","key":"D"}]
E
https://exam-online.oss-cn-beijing.aliyuncs.com/2106a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106d.png/>,key:D}]、[{"value":" class="import-img" ","key":"A"},{"value":" class="import-img" ","key":"B"},{"value":" class="import-img" ","key":"C"},{"value":" class="import-img" ","key":"D"}]
F
https://exam-online.oss-cn-beijing.aliyuncs.com/2106a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2106d.png/>,key:D}]、[{"value":" class="import-img" ","key":"A"},{"value":" class="import-img" ","key":"B"},{"value":" class="import-img" ","key":"C"},{"value":" class="import-img" ","key":"D"}]
正确答案:
Explanation 选项A的变换是不正确的,或运算不能如此变换。选项B的变换是正确的。选项C的变换是不正确的,投影出A1,B2,B3后不能实现选择操作。选项D的变换是不正确的,移到乘积下面的投影存在着E1,E2所没有的属性。
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