问题 单项选择题

下列关于关系运算的叙述中正确的是

A) 投影、选择、连接是从二维表的行的方向来进行运算
B) 并、交、差是从二维表的列的方向来进行运算
C) 投影、选择、连接是从二维表的列的方向来进行运算
D) 以上三种说法都不对

答案

参考答案:C

解析: 在关系模型的数据语言中,一般除了运用常规的集合运算(并、交、差、笛卡尔积等)外,还定义了一些专门的关系运算,如投影、选择、连接等运算。前者是将关系(即二维表)看成是元组的集合,这些运算主要是从二维表的行的方向来进行的;后者主要是从二维表的列的方向来进行运算。因此,选项A与B中的说法都是错误的,而选项C中的说法是正确的。

单项选择题

The term "nationalism" is generally used to describe two phenomena: (1)the members of a nation care about their national identity and(2) that the members of a nation seek to achieve (or sustain) self-determination.

It is traditional, therefore, to distinguish nations from states—whereas a nation often consists of an ethnic or cultural community, a state is a political entity with a high degree of sovereignty. While many states are nations in some sense, there are many nations which are not fully sovereign states. As an example, the Native American Iroquois constitute a nation but not a state, since they do not possess the requisite political authority over their internal or external affairs. If the members of the Iroquois nation were to strive to form a sovereign state in the effort to preserve their identity as a people, they would be exhibiting a state—focused nationalism.

Nationalism has long been ignored as a topic in political philosophy, written off as a relic from bygone times. It has only recently come into the focus of philosophical debate. The surge of nationalism usually presents a morally ambivalent and for this reason often fascinating picture. "National awakenings" and struggles for political independence are often both heroic and inhumanly cruel; the formation of a recognizably national state often responds to deep popular sentiment, but can and does sometimes bring in its wake inhuman consequences, including violent expulsion and "cleansing" of non-nationals, all the way to organized mass murder. The moral debate on nationalism reflects a deep moral tension between solidarity with oppressed national groups on the one hand and repulsion in the face of crimes perpetrated in the name of nationalism on the other.

Nationalism may manifest itself as part of official state ideology or as a popular (non-state) movement and may be expressed along civic, ethnic, cultural, religious or ideological lines. These self-definitions of the nation are used to classify types of nationalism. However, such categories are not mutually exclusive and many nationalist movements combine some or all of these elements to varying degrees. Nationalist movements can also be classified by other criteria, such as scale and location.

Nationalism does not necessarily imply a belief in the superiority of one race over others, but in practice, many nationalists support racial protectionism or racial supremacy. Such racism is typically based upon preference or superiority of the indigenous race of the nation.

The example of Native American Iroquois is used in paragraph 2 to ()

A. show the differences between nations and states

B. discuss the differences between internal and external affairs

C. indicate their strive to form a state

D. present their identity as a people

单项选择题