问题 单项选择题

在如下2个数据库的表中,若雇员信息表EMP的主键是雇员号,部门信息表DEPT的主键是部门号。若执行所列出的操作,哪一项操作不能执行

答案

参考答案:D

解析:解析:当执行删除操作时,一般只需要检查参照完整性规则。如果是删除被参照关系中的行,检查被删除行在主码属性上的值是否正在被相应的被参照关系的外码引用,若不被引用,可以执行删除操作;若被引用,有3种可能的做法:不可以执行删除操作(拒绝删除),或将参照关系中相应行在外码属性上的值改为空值后再执行删除操作(空值删除),或将参照关系中相应行一起删除(级联删除)。

填空题
阅读理解

China is going green. In order to reduce air pollution and oil shortages, automobile manufacturers have announced their plans to develop hybrid vehicles (混合燃料汽车) for the Chinese market. Toyota's hybrid car Prius will be ready to drive in China this week. Let's have a look at the new car.

Any vehicle is a hybrid when it combines two or more sources of power. Hybrid cars run off a rechargeable battery and gasoline. Hybrid cars have special engines, which are smaller than traditional gasoline engines. They run at 99 percent of their power when the car is cruising (匀速行驶). A specially designed battery motor provides extra power for running up hills or when extra acceleration is needed.

Step into a Prius, and turn on the engine. The first thing you notice is how much quieter it is than a traditional car. At this point, the car's gasoline engine is dormant (休眠). The electric motor will provide power until the car reaches about 24 km/h. If you stay at a low speed, you are effectively driving an electric car, with no gasoline being used, and no waste gas gives off.

The onboard (车载的) computer makes the decision about when to use a gas engine, when to go electric, and when to use a combination of the two. If you go over 24 km/h, when you step on the gas pedal (油门), you are actually telling the computer how fast you want to go.

The electronic motor recharges automatically using a set of batteries. When driving at high speed, the gasoline engine not only powers the car, but also charges the batteries. Any time you use the brake, the electric motor in the wheels will work like a generator and produce electricity to recharge the batteries. As a consequence, the car's batteries will last for around 200,000 miles.

小题1:The author writes this passage mainly to________.

A.teach people how to drive a hybrid car

B.introduce a new kind of “green” car

C.show how to save their gasoline when driving a car

D.announce plans to develop hybrid vehicles for China小题2:Which of the followings is NOT true?

A.The hybrid cars reduce air pollution and oil shortages.

B.The car's gasoline engine doesn't work until it reaches about 24 km/h.

C.This kind of car is completely controlled by an onboard computer.

D.A specially designed battery motor provides extra power when needed.小题3:What is the most important feature of hybrid cars?

A.They are powered by both a rechargeable battery and gasoline.

B.They are much quieter than traditional cars.

C.They only use 99 percent of their power to run up hills.

D.They have smaller engines than traditional gasoline ones.小题4:What can we conclude from the story?

A.The batteries can be recharged at any time you want.

B.Using the brake suggests that the gasoline engine should work.

C.The batteries will last for 200, 000 miles without being recharged.

D.The gasoline engine charges the batteries as well as powers the car.