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进程P_A不断地向管道写数据，进程P_B从管道中读数据并加工处理，如下图所示。如果采用PV操作来实现进程P_A和进程P_B间的管道通信，并且保证这两个进程并发执行的正确性，则至少需要（）

若有关系模式R(A,B,C)和S(C,D,E)，对于如下的关系代数表达式：
E₁=π_A,D(σ_{B＜’2003’^R.C=S.C^E=’80’}(R×S)) E₂=π_A,D(σ_R.C=S.C(σ_{B＜’2003’}(R)×σ_E=’80’(S)))
E₃=π_A,D(σ_{B＜’2003’}(R)σ_E=’80’(S)) E₄=π_A,D(σ_{B＜’2003’^E=’80’}(RS))
正确的结论是（1），表达式（2）的查询效率最高。

若有关系模式R(A,B,C)和S(C,D,E)，对于如下的关系代数表达式：
E₁=π_A,D(σ_{B＜’2003’^R.C=S.C^E=’80’}(R×S)) E₂=π_A,D(σ_R.C=S.C(σ_{B＜’2003’}(R)×σ_E=’80’(S)))
E₃=π_A,D(σ_{B＜’2003’}(R)σ_E=’80’(S)) E₄=π_A,D(σ_{B＜’2003’^E=’80’}(RS))
正确的结论是（1），表达式（2）的查询效率最高。

设有关系模式S(Sno, Sname, Pno, Pname, Q, A)表示销售员销售商品情况，其中各属性的含义是：Sno为销售员员工号，Sname为销售员姓名，Pno为商品号，Pname为商品名称，Q为销售商品数目，A为销售商品总金额，根据定义有如下函数依赖集：P=Sno→Sname，Sno→Q，Sno→A，Pno→Pname
关系模式S的关键字是（1），W的规范化程度最高达到（2）。若将关系模式S分解为3个关系模式$1(Sno, Sname, Q, A), S2(Sno, Pno, Pname)，则S1的规范化程度最高达到（3），S2的规范化程度最高达到（4）。
SQL中集合成员资格的比较操作“元组IN(集合)”中的“IN”与（5）操作符等价。

A Web browser is simply a terminal emulator, designed to display text on a screen. The two essential differences between an ordinary terminal emulator and a Web browser are that the browser knows how to deal with （1）, and that it has a mechanism for（2） graphical files. Display text, display graphics, and （3） hyperlinks--there’s 99 percent of the （4） value. That’s not to say that the manufacturers didn’t go all-out to attach a hyperactive efflorescence of useless capabilities onto their browsers. Remember when media channels in the browser were a big deal, instead of the clutter you can’t wait to delete from your favorites of bookmarks menu Remember when client-side Java applets were supposed to become the preferred（5）for application development Remember frames and all their nasty side effects

A Web browser is simply a terminal emulator, designed to display text on a screen. The two essential differences between an ordinary terminal emulator and a Web browser are that the browser knows how to deal with （1）, and that it has a mechanism for（2） graphical files. Display text, display graphics, and （3） hyperlinks--there’s 99 percent of the （4） value. That’s not to say that the manufacturers didn’t go all-out to attach a hyperactive efflorescence of useless capabilities onto their browsers. Remember when media channels in the browser were a big deal, instead of the clutter you can’t wait to delete from your favorites of bookmarks menu Remember when client-side Java applets were supposed to become the preferred（5）for application development Remember frames and all their nasty side effects

设有关系模式S(Sno, Sname, Pno, Pname, Q, A)表示销售员销售商品情况，其中各属性的含义是：Sno为销售员员工号，Sname为销售员姓名，Pno为商品号，Pname为商品名称，Q为销售商品数目，A为销售商品总金额，根据定义有如下函数依赖集：P=Sno→Sname，Sno→Q，Sno→A，Pno→Pname
关系模式S的关键字是（1），W的规范化程度最高达到（2）。若将关系模式S分解为3个关系模式$1(Sno, Sname, Q, A), S2(Sno, Pno, Pname)，则S1的规范化程度最高达到（3），S2的规范化程度最高达到（4）。
SQL中集合成员资格的比较操作“元组IN(集合)”中的“IN”与（5）操作符等价。

A Web browser is simply a terminal emulator, designed to display text on a screen. The two essential differences between an ordinary terminal emulator and a Web browser are that the browser knows how to deal with （1）, and that it has a mechanism for（2） graphical files. Display text, display graphics, and （3） hyperlinks--there’s 99 percent of the （4） value. That’s not to say that the manufacturers didn’t go all-out to attach a hyperactive efflorescence of useless capabilities onto their browsers. Remember when media channels in the browser were a big deal, instead of the clutter you can’t wait to delete from your favorites of bookmarks menu Remember when client-side Java applets were supposed to become the preferred（5）for application development Remember frames and all their nasty side effects

设有关系模式S(Sno, Sname, Pno, Pname, Q, A)表示销售员销售商品情况，其中各属性的含义是：Sno为销售员员工号，Sname为销售员姓名，Pno为商品号，Pname为商品名称，Q为销售商品数目，A为销售商品总金额，根据定义有如下函数依赖集：P=Sno→Sname，Sno→Q，Sno→A，Pno→Pname
关系模式S的关键字是（1），W的规范化程度最高达到（2）。若将关系模式S分解为3个关系模式$1(Sno, Sname, Q, A), S2(Sno, Pno, Pname)，则S1的规范化程度最高达到（3），S2的规范化程度最高达到（4）。
SQL中集合成员资格的比较操作“元组IN(集合)”中的“IN”与（5）操作符等价。

设有关系模式S(Sno, Sname, Pno, Pname, Q, A)表示销售员销售商品情况，其中各属性的含义是：Sno为销售员员工号，Sname为销售员姓名，Pno为商品号，Pname为商品名称，Q为销售商品数目，A为销售商品总金额，根据定义有如下函数依赖集：P=Sno→Sname，Sno→Q，Sno→A，Pno→Pname
关系模式S的关键字是（1），W的规范化程度最高达到（2）。若将关系模式S分解为3个关系模式$1(Sno, Sname, Q, A), S2(Sno, Pno, Pname)，则S1的规范化程度最高达到（3），S2的规范化程度最高达到（4）。
SQL中集合成员资格的比较操作“元组IN(集合)”中的“IN”与（5）操作符等价。

A Web browser is simply a terminal emulator, designed to display text on a screen. The two essential differences between an ordinary terminal emulator and a Web browser are that the browser knows how to deal with （1）, and that it has a mechanism for（2） graphical files. Display text, display graphics, and （3） hyperlinks--there’s 99 percent of the （4） value. That’s not to say that the manufacturers didn’t go all-out to attach a hyperactive efflorescence of useless capabilities onto their browsers. Remember when media channels in the browser were a big deal, instead of the clutter you can’t wait to delete from your favorites of bookmarks menu Remember when client-side Java applets were supposed to become the preferred（5）for application development Remember frames and all their nasty side effects

设有关系模式S(Sno, Sname, Pno, Pname, Q, A)表示销售员销售商品情况，其中各属性的含义是：Sno为销售员员工号，Sname为销售员姓名，Pno为商品号，Pname为商品名称，Q为销售商品数目，A为销售商品总金额，根据定义有如下函数依赖集：P=Sno→Sname，Sno→Q，Sno→A，Pno→Pname
关系模式S的关键字是（1），W的规范化程度最高达到（2）。若将关系模式S分解为3个关系模式$1(Sno, Sname, Q, A), S2(Sno, Pno, Pname)，则S1的规范化程度最高达到（3），S2的规范化程度最高达到（4）。
SQL中集合成员资格的比较操作“元组IN(集合)”中的“IN”与（5）操作符等价。

A Web browser is simply a terminal emulator, designed to display text on a screen. The two essential differences between an ordinary terminal emulator and a Web browser are that the browser knows how to deal with （1）, and that it has a mechanism for（2） graphical files. Display text, display graphics, and （3） hyperlinks--there’s 99 percent of the （4） value. That’s not to say that the manufacturers didn’t go all-out to attach a hyperactive efflorescence of useless capabilities onto their browsers. Remember when media channels in the browser were a big deal, instead of the clutter you can’t wait to delete from your favorites of bookmarks menu Remember when client-side Java applets were supposed to become the preferred（5）for application development Remember frames and all their nasty side effects