问题 单项选择题

若对关系R(A,B,C,D)和S(C,D,E)进行关系代数运算,则表达式π3,4,7(σ4<5(R×S))与()等价。

A.πC,D,ED<C(R×S))

B.πR,C,R,D,ER.D<S.C(R×S))

C.πC,D,ER.D<S.C(R×S))

D.πR,C,R,D,ED<C(R×S))

答案

参考答案:B

解析:

本题考查关系代数运算方面的基础知识。

题目要求计算关系代数表达式π3,4,74<5(R×S))的结果集,其中,R×S的属性列名分别为:R.A,R.B,R.C,R.D,S.C,S.D和S.E,σ4<5(R×S)的含义是从R×S结果集中选取第4个分量(R.D)小于第5个分量(S.C)的元组,故σ4<5(R×S)与σR.D<S.C(R×S)等价。π3,4,74<5(R×S))的含义是从σ4<5(R×S)结果集中选取第3列R.C、第4列R.D,和第7列S.E(或E),故π3,4,74<5(R×S))与πR,C,R,D,ER.D<S.C(R×S))等价。需要说明的是第3列R.C不能简写为C,因为关系S的第一列属性名也为C,故必须标上关系名加以区别;同样,第4列R.D也不能简写为D,因为关系S的第2列属性名也为D,故必须标上关系名加以区别。

选择题
阅读理解

A long weekend is a great opportunity to do something a little different. Many people use a long weekend for a getaway, a short trip to leave the stress of their everyday lives behind and explore, learn, or relax.

Hotel Romance

Many hotels and resorts feature something special to start the getaway, such as champagne, flowers, or a fruit basket in the room on arrival. Getaway packages usually include a room with something special, a heart shaped bed, for example. Additional benefits could be room service meals or other private dining, plus tickets or discounts for local attractions.

Visit a Spa

Spa getaways can take place at a location that focuses only on spa services or at a hotel that offers a spa as one of its features. Some spas emphasize treatments fit for their geographic area or a local attraction. A spa in a grape growing region might provide grape juice masks, or a hotel near a hot spring could offer mineral baths. Most spas offer a range of massage, health and beauty treatments, so a spa getaway can be relaxing with other benefits.

Learn a Skill

A long weekend may not be enough time to become an expert at something new, but it provides a way for people to begin their explorations. Some organizations offer intensive courses that last two or three days. People with special interests can get away from their routines while learning such things as how to make beer, speak a new language, or take better photos.

Help Others

Some organizations cooperate with businesses to have accommodations (住宿) and activities for short-term volunteers. Most volunteer vacations involve ecology friendly or charity activities. A volunteer getaway is a way to relieve stress and help others at the same time.

Stay in a National Park

For those who want to relax in a rural environment, a national park fits the bill. Most national parks provide camping areas for tents and trailers. Several have cabins (小屋) within the parks available to rent. A getaway to a national park provides privacy, stress relief, and the time to get to know some important natural resources.

小题1:The underlined word getaway in this passage most probably means      .

A.exploration

B.location

C.vacation

D.activity小题2: It can be learned from the text that       .

A.spas are offered in the tourist attractions all over the country

B.intensive courses of some organizations help people get away from home

C.helping others at weekend makes people feel free and relaxed

D.people may get close to nature in a national park小题3:Which of the following can be rented in a national park?

A.Cabins.

B.Trailers.

C.Camping area.

D.Privacy.小题4:The best title of the passage can be         .

A.Away from Our Busy Work

B.Good Ways to Relieve Stress

C.Enjoy Life and Help Others

D.Long Weekend Getaway Ideas