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在线名词解释大全 - 数据库系统 - 详细

“SELECTSname,Cname,ScoreFROMStudent,Course,SCWHERECteacher=‘张杰’andStudent.S#=SC.S#andCourse.C#=SC.C#”请用语法树给出其最后的优化结果，正确的是____________。已知关系Student(S#,Sname,Sage,Sclass)，Course(C#,Cname,Credit,Cteacher)，SC(S#,C#,Score)。给定SQL语句如下：

A

https://exam-online.oss-cn-beijing.aliyuncs.com/2110a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110d.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/2110a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110d.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/2110a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110d.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/2110a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110d.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/2110a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110d.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/2110a.png/>,key:A},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110b.png/>,key:B},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110c.png/>,key:C},{value:、https://exam-online.oss-cn-beijing.aliyuncs.com/2110d.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，这是未优化的语法树。选项B，这是“尽可能将‘选择’操作移动到树的底部的结果，还需要进一步优化。选项C，这不是优化后的最终结果，还需要将乘积操作和其后的选择操作组合连接操作。选项D，这是优化后的最终结果。

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