Arelation（from the relational database model)consistsof（由⋯⋯组成)a set of tuples（元组),whichimplies（意味着)that（).

A.all tuples in a relation must be distinct(独一无二的)

B.every relation has a key(键)

C.all of the above

D.for any two tuples,the values associated with one or more of thei rattributes(属性)must differ

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Every value in arelation must be（).
A.composite(复合的)and single-valued
B.single-valued
C.notnull and single-valued
D.simple and single-valued

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在 oracle 中有表 t，从表 t 中获取第 31 到 40 的记录，下列哪写语句是正确的（）

设有关系R（A,B,C)和S（C,D)。与SQL语句select A,B,D from R,S where R.C=S.C等价的关系代数表达式
设有关系R(A,B,C)和S(C,D)。与SQL语句select A,B,D from R,S where R.C=S.C等价的关系代数表达式是______。
A．σR.C=S.C(πA,B,D(R×S))
B．πA,B,D(σR.C=S.C(R×S))
C．σR.C=S.C((πA,BR)×(πDS))
D．σR.C=S.C(πD(πA,BR)×S)

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考虑赫克索尼亚经济。 a．消费函数是：C=200＋0.75（Y－T)；投资函数是：I=200－25r：政府购买和税收都是100，针对这

考虑赫克索尼亚经济。
a．消费函数是：C=200+0.75(Y-T)；投资函数是：I=200-25r：政府购买和税收都是100，针对这个经济体，画出r在0～8之间的IS曲线。
b．在赫克索尼亚，货币需求函数是：(M/P)^d=Y-100r：货币供给M是1000，物价水平P是2，针对这个经济，画出r在0～8之间的LM曲线。
c．找出均衡利率r和均衡收入水平Y。
d．假定政府购买从：100增加到150，IS曲线会移动多少？新的均衡利率和收入水平是多少？
e．假设货币供给从1000增加到1200，LM曲线会移动多少？新的均衡利率和收入水平是多少？
f．使用货币和财政政策的初始值，假设物价水平从2上升到4，会发生什么变化？新的均衡利率和收入水平是多少？
g．推导并画出总需求方程式。如果财政或货币政策像(d)和(e)部分那样变动，这条总需求曲线会发生什么变动？
Consider the economy of Hicksonia.
a．The consumption function is given by C=200+0.75(Y-T). The investment function is I=200-25r. Government purchases and taxes are both 100. For this economy, graph the IS curve for r ranging from 0 t0 8.
b．The money demand function in Hicksonia is (M/P)^d=Y-100r. The money supply M is 1,000 and the price level P is 2. For this economy, graph the LM curve for r ranging from 0 to 8.
c．Find the equilibrium interest rate r and the equilibrium level of income Y.
d．Suppose that government purchases are raised from 100 to 150. How much does the IS curve shift？ What are the new equilibrium interest rate and level of income？
e．Suppose instead that the money supply is raised from 1,000 to 1,200. How much does the LM curve shift？ What are the new equilibrium interest rate and level of income？
f．With the initial values for monetary and fiscal policy, suppose that the price level rises from 2 to 4. What happens？ What are the new equilibrium interest rate and level of income？
g．Derive and graph an equation for the aggregate demand curve. What happens to this aggregate demand curve if fiscal or monetary policy changes, as in parts (d) and (e)？

结构化查询语言SQL是一种（41)语言，其主要功能有（42)，它的典型句型为 SELECT A1，A2，…，An FROMr1，r
结构化查询语言SQL是一种(41)语言，其主要功能有(42)，它的典型句型为
SELECT A1，A2，…，An
FROM r1，r2，…，rm
WHERE F
其中A1，A2，…，An、r1，r2，…，rm和F分别是(43)。
关系模式SCS(Sno，Cno，Score)中，Sno是学生的学号，Cno是课程号，Score为成绩。若要查询每门课程的平均成绩，且要求查询结果按平均成绩升序排列。平均成绩相同时，按课程号降序排列，可用SQL语言写为(44)。若查询结果仅限于平均分数超过85分的，则应(45)。
A．人工智能
B．关系数据库
C．函数型
D．高级算法

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Survey results indicate that smoking and alcohol and marijuana（大麻) use increased among r
Survey results indicate that smoking and alcohol and marijuana(大麻) use increased among residents of Manhattan during the 5～8 weeks after the terrorist attacks on the World Trade Center which took place on September 11, 2001. Almost one third of the nearly 1,000 persons interviewed reported an increased use of alcohol, marijuana, or cigarettes following the September 11th attacks. The largest increase was in alcohol use. 1About one fourth of the respondents said they were drinking more alcohol in the weeks after September 11; about 10% reported an increase in smoking, and 3.2% said they had increased their use of marijuana.
The investigators found survey participants by randomly dialing New York City phone numbers and screened (筛选) potential respondents for Manhattan residents living in areas close to the World Trade Center. Interviews were conducted with 988 individuals between October 16 and November 15, 2001. Participants were asked about their cigarette smoking, alcohol drinking, and marijuana use habits before and after September 11. During the week prior to September 11, 2001, 22.6% of the participants reported smoking cigarettes, 59.1% drinking alcohol, and 4.4% using marijuana. After September 11th, 23.4% reported smoking cigarettes, 64.4% drinking alcohol, and 5.7% smoking marijuana. Among those who smoked, almost 10% reported smoking at least an extra pack of cigarettes a week and among those who drank alcohol, more than 20% reported drinking at least one extra drink a day.
The researchers found that people who reported an increase in substance abuse were more likely to suffer from post traumatic(外伤的)stress disorder (PTSD) and from depression. People who reported an increase in cigarette smoking or marijuana use were also more likely to have both P1SD and depression, while those who reported an increase in alcohol use were more likely to have depression only. Persons who were living closer to the World Trade Center were more likely to increase their cigarette smoking, but other factors such as being displaced from home, losing possessions during the attacks, or being involved in the rescue efforts were not consistently associated with increased substance use. Symptoms of panic attack were associated with an increase in the use of all substances.
Increase in substance abuse did not differ significantly between men and women or among racial or ethnic groups. Demographic(人口统计学的) factors such ms age, marital status, and income seemed to play a more critical role in determining if the events of September 11th led to an increase in substance use.
The survey results suggest that the largest increase in substance use was ______.
A．alcohol
B．marijuana
C．cigarettes
D．cocaine

用…制造的英语是（）

下列微粒半径之比大于1的是（）

已知r=xex+yey+zez,r=（x2+y2+z2)1/2，试证： （1)▽×r=0 （2) （3)（f（r)是r的函数)
已知r=xe_x+ye_y+ze_z,r=(x²+y²+z²)^1/2，试证：
(1)▽×r=0
(2)
(3)(f(r)是r的函数)

>空气密度ρ与比重r的关系是（）