问题 填空题

视图是______的表,其内容是根据查询定义的。

答案

参考答案:虚拟

解析: 视图是虚拟的表,其内容是根据查询定义的。和实际的表一样,视图由一系列字段和记录组成,但不同的是,视图的数据不是数据库中实际存储的数据集合。视图的记录和字段是根据视图定义,查询所引用的表导出的,因此,视图在引用时动态产生数据。视图是表的过滤器,视图提供了查看表中数据的另外一种方法。表是用来存储具体的物理介质上数据的结构,而视图数据在物理上并不存在。视图是存储在系统目录中的信息,是从一个或多个表中派生出来的,这些信息是和物理存储的表有关的唯一信息,因此,视图又称虚拟表。

单项选择题
阅读理解

At least 10 million hectares of cultivated(耕作)land in China are polluted, which makes a grave threat to the country’s food safety, the Xinhua News Agency reported on Friday.

The polluted land,which is mostly in economically developed areas,accounts for one-tenth of the country’s total arable(可耕种的)land,according to an incomplete survey by the State Environmental Protection Administration,China’s top environment watchdog.

Pollution on cultivated land threatens the environment,food safety and the sustainable development of agriculture,the administration said.

Because of continual,too much use of chemical fertilizer, pesticide and agricultural plastic sheeting, as well as irrigation using polluted water, a large amount of contaminants(污染物)remain in the cultivated land.

The contaminants affect the soil’s ecological structure and function,leading to decreased soil productivity, lower crop yield(产量),and lower quality of agricultural products-

It also leads to worse water quality in rural areas.

Less than 9 per cent of drinkable water passed checks for bacteria in243 rural water supply stations across the county.

Another survey, which took samples of drinking water and groundwater an 69 small towns。 in Beijing, Tianjin and Hebei Province of North,China, showed that more than half of the water contains too much nitrate(硝酸盐),which may cause diabetes(糖尿病)and damage the kidney.

The administration estimated at least 1 90 million farmers are drinking water that contains harmful substances.

Many villagers drink unfiltered water taken from shallow water wells or water cellars, which are reported to have poor sanitary conditions·

小题1:The author writes the passage to            .

A.1et the readers examine their mistakes

B.present the readers a new idea

C.come up with a solution to ending farmland pollution

D.inform the readers of the importance of fighting farmland pollution小题2:The underlined word “grave” in Paragraph 1 probably means

A.serious

B.certain

C.big

D.terrible小题3:Pollution on cultivated land leads to the following outcomes EXCEPT       .

A.a threat to China’s food safety

B.worse water quality in rural areas

C.a variety of diseases

D.the sustainable development of agriculture