题目内容

关系型数据库的关系数据模型通常由( )组成。

A. 数据结构、数据通信、关系操作
B．数据结构、关系操作、完整性约束
C. 数据通信、关系操作、完整性约束
D. 数据结构、数据通信、完整性约束

查看答案

搜索结果不匹配？点我反馈

更多问题

We hope that by observing the following points you will enjoy your visit to Christ Church without disturbing the life of the college.* Please obey all notices and do not enter the areas marked Private.* Please do not enter any college rooms.* Please avoid leaving litter (杂物) ; picnicking within the college is not permitted.* Please do not smoke.* Please do not gather so as to obstruct (阻塞) paths or passages, particularly in the Hall.* Please be as quiet as possible, remembering that this is a college where people are working.* In the event of a fire or other danger, or if you hear a fire alarm or warning, please leave the building without delay. At all times please follow the advice of the Custodians (保安人员) who are here to help you.* Closed circuit television surveillance (监视) is in operation. Images are being recorded for purposes of crime prevention and public safety.* Thank you for visiting Christ Church. If you have any comments please write to:The Steward, Christ Church, Oxford OX1 1DP What is not permitted to do during the visit to the collegeLeaving litter, _________________________inside the college or smoking, etc.

We hope that by observing the following points you will enjoy your visit to Christ Church without disturbing the life of the college.* Please obey all notices and do not enter the areas marked Private.* Please do not enter any college rooms.* Please avoid leaving litter (杂物) ; picnicking within the college is not permitted.* Please do not smoke.* Please do not gather so as to obstruct (阻塞) paths or passages, particularly in the Hall.* Please be as quiet as possible, remembering that this is a college where people are working.* In the event of a fire or other danger, or if you hear a fire alarm or warning, please leave the building without delay. At all times please follow the advice of the Custodians (保安人员) who are here to help you.* Closed circuit television surveillance (监视) is in operation. Images are being recorded for purposes of crime prevention and public safety.* Thank you for visiting Christ Church. If you have any comments please write to:The Steward, Christ Church, Oxford OX1 1DP What is the name of the place open to touristsIt’s______________________________________.

Historians tend to tell the same joke when they are describing history education in America. It’’s the one (61) the teacher standing in the schoolroom door (62) goodbye to students for the summer and calling (63) them, "By the way, we won World War II." The problem with the joke, of course, is that it’’s (64) funny. The recent surveys on (65) illiteracy (无知) are beginning to numb (令人震惊): nearly one third of American 17-year-olds cannot even (66) which countries the United States (67) against in that war. One third have no (68) when the Declaration of Independence was (69) . One third thought Columbus reached the New World after 1750. Two thirds cannot correctly (70) the Civil War between 1850 and 1900. (71) when they get the answers right, some are (72) guessing. Unlike math or science, ignorance of history cannot be (73) connected to loss of international (74) But it does affect our future (75) a democratic nation and as individuals. The (76) news is that there is growing agreement (77) what is wrong with the (78) of history and what needs to be (79) to fix it. The steps are tentative (尝试性的) (80) yet to be felt in most classrooms.

Fear and its (36) ___pain are two of the most useful things that men and animals (37)_, if they are properly used. If fire did not hurt when it burnt, children would play it until their hands were burnt away. (38) _, if pain existed but fear didn’’t, a child would bum itself again and again, because fear would not warn it to keep away from the fire that had burnt before. Fear and pain are (39) _two guards without which men and animals might soon die out. We suggest fear be properly used. If, for example, you never go out of your house because of the danger of being (40) _down and killed in the street by a car, you are letting fear rule you too much. Even in your house you are not (41) _safe: an airplane may (42) _on your house. The important thing is not to let feat rule you, but (43)) _to use fear as your servant and guide. In many cases, (44)______. For example, you see a car coming straight towards you; fear warns you, you jump out of the way, and all is well. (45)_. In this case, fear has given you its warning; you have examined it and decided on your course of action, (46)________.