问题 问答题


阅读以下说明和关系表,回答问题1~3。
[说明]
设T1,T2,T3是如下三个事务,其中S为数据库中某个数据项,设S的初值为1。
T1: S:= S * S
T2: S:= S + 3
T3: S:= 6

[问题3]
一个事务应该具有哪些属性请具体说明。

答案

参考答案:

解析:事务的四个性质是:原子性、一致性、隔离性和持久性。原子性:是保证数据库系统完整性的基础。一个事务中所有对数据库的操作是一个不可分割的操作序列。一致性:一个事务独立执行的结果将保证数据库的一致性,即数据不会因事务的执行而遭受破坏。隔离性:隔离性要求在并发事务被执行时,系统应保证与这些事务先后单独执行时结果一样,使事务如同在单用户环境下执行一样。持久性:要求对数据库的全部操作完成后,事务对数据库的所有更新应永久地反映在数据库中。

阅读理解

Cheating can happen in a lot of different ways. When people cheat, it’s not fair to other people, like the kids who studied for the test or who were the true winners of a game.

Many people like the action of cheating. It makes difficult things seem easy, like getting all the right answers on the test. But it doesn’t solve the problem of not knowing the material and it won’t help on the next test --- unless the person cheats again.

Some people lose respect for cheaters and think less of them. The cheaters themselves may feel bad because they know they are not really earning that good grade. And, if they get caught cheating, they will be in trouble at school, and maybe at home, too.

Some kids cheat because they’re busy or lazy and they want to get good grades without spending the time studying. Other kids might feel like they can’t pass the test without cheating. Even when there seems to be a “good reason” for cheating, cheating isn’t a good idea.

If you were sick or upset about something the night before and couldn’t study, it would be better to talk with the teacher about this. And if you don’t have enough time to study for a test because of swim practice, you need to talk with your parents about how to balance swimming and school. A kid who thinks cheating is the only way to pass a test needs to talk with the teacher and his or her parents so they can find some solutions together. Talking about these problems and working them out will feel better than cheating.(291 words)

小题1:Some students like cheating mainly because_________.

A.the material in the test is very difficult

B.they want to do better than the others

C.cheating can make hard things seem very easy

D.they have little time to study their lessons小题2:We can learn from the passage______.

A.cheaters are often thought highly of

B.people show no respect for those who cheat

C.parents whose kids cheat are often in trouble

D.kids cheat in the test because of swim practice小题3:In the passage those cheaters are advised to_______.

A.spend more time on school than on sports

B.find good solutions instead of cheating

C.try hard to be intelligent rather than lazy

D.ask their classmates for good methods of study

判断题