关系代数中有9种操作，其中，对关系进行垂直分割，消去某些列，并重新安排列的顺序的操作是(51)；根据某些关系进行水平分割的操作是(52)。在操作(53)中，选择两个关系的公共属性均相等的元组，并去掉该操作中重复的公共属性列，将得到一种新的操作(54)，如果两个关系没有公共属性，则这种新的操作就转化成(55)。设有关系R(A，B，C)和S(B， C，D)，下列关系代数表达式中不成立的是(56)。

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Because　Web　servers　axe　platform.　and　application(71),　they　can　send　or　request　data　from　legacy　or　external　applications　including　databases.　All　replies,　once　converted　into(72).　mark-up　language,　can　then　be　transmitted　to　a(73).Used　in　this　way,　Intranets　can(74)lower　desktop　support　costs,　easy　links　with　legacy　applications　and　databases　and,(75)all,　ease　of　use.

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Prior　to　the　UML,　there　was　no　clearleading(66)Ianguage.　Users　had　to　choose　from　among　many　similar　modeling　languages　with　minor　differences　in　overall(67)power.　Most　of　the　modeling　languages　shared　a　set　of　commonly　accepted　concepts　that　are　expressed　slightly　differently　in　various　languages.　This　lack　of(68)discouraged　new　users　from　entering　the　OO　market　and　from　doing　OO　modeling,　without　greatly　expanding　the　power　of　modeling.　Users　longed　for　the　industry　to　adopt　one,　or　a　very　few,　broadly　supported　modeling　languages　suitable　for(69)usage.

Some　Vendors　were　discouraged　from　entering　the　OO　modeling　area　because　of　the　need　to　support　many　similar,　but　slightly　different,　modeling　languages.　In　particular,　the　supply　of　add-on　tools　has　been　depressed　because　small　vendors　cannot　afford　to　support　many　different　formats　from　many　differem(70)modeling　tools.　It　is　important　to　the　entire　OO　industry　to　encourage　broadly　based　tools　and　vendors,　as　well　as　niche　products　that　cater　to　the　needs　of　specialized　groups.

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