2. 计算机
  3. One　of　the　difficulties　in　building　an　SQL,like　query　language　for　the　Web　is　the　absence　of　a　database(71)　for　this　huge,　heter

One　of　the　difficulties　in　building　an　SQL,like　query　language　for　the　Web　is　the　absence　of　a　database(71)　for　this　huge,　heter

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问题 One　of　the　difficulties　in　building　an　SQL,like　query　language　for　the　Web　is　the　absence　of　a　database(71)　for　this　huge,　heterogeneous　repository　of　information.　However,　if　we　are　interested　in　HTML　documents　only,　we　can　construct　a　virtual　schema　from　the　implicit　structure　of　these　files.　Thus,　at　the　highest　level　of(72)　,　every　such　document　is　identified　by　its　Uniform　Resource　Locator(URL),　has　a(73)　and　a　text.　Also,　Web　servers　provide　some　additional　information　such　as　the　type,　length,　and　the　last　modification　date　of　a　document.　So,　for　data　mining　purposes,　we　can　consider　the　set　of　all　HTML　documents　as　a　relation:Document(url,　title,　text,　type,　length,　modif)Where　all　the(74)　are　character　strings.　In　this　framework,　an　individual　document　is　identified　with　a(75)　in　this　relation.　Of　course,　if　some　optional　information　is　missing　from　the　HTML　document,　the　associate　fields　will　be　left　blank,　but　this　is　not　uncommon　in　any　database.
选项 A、relation
B、field
C、script
D、tuple
答案D
解析
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