问题 填空题

[问题1] 如果将上述应用的数据库设计成如下关系模式; RS (A#,A1,A2,A3,B#,B1,B2,D1),请指出该关系模式的候选键。

答案

参考答案:(A#,B#)

解析:
[解答要点] 根据候选关键字的定义:设关系模式R (A1,A2,…,An),F是R上的函数依赖集。X是R的一个子集,
①x→A1A2…An∈F+(它的意思是X能够决定唯一的一个元组)。
②不存在X的真子集Y,使得Y也能决定唯一的一个元组,则X就是R的一个候选键(它的意思是,X能决定唯一的一个元组但又没有多余的属性集。)由于(A#,B#)+=(A#,A1,A2,A3,B#,B1,B2,D1),即 (A#,B#)→(A#,A1,A2,A3,B#,B1,B2,D1)∈F+,而且(A#)+=(A#,A1,A2,A3),(B#)+=(B#,B1,B2)。因此,(A#,B#)是关系模式贴的候选键。
假设第二问的关系模式RS上存在函数依赖:A1→A3,则贴只满足1NF,不满足2NF,因为存在非主属性A3对键(A#,B#)的传递依鞍,而且存在非主属性对键的部分函数依赖。
在第三问中,因为R2中有两个非主属性B1和B2,候选键是B#,既不存在非属性对候选键的部分函数依赖,也不存在非主属性对候选键的传递函数依赖。

阅读理解

William Butler Yeats, a most famous Irish writer, was born in Dublin on June 13, 1865. His childhood lacked the harmony that was typical of a happy family. Later, Yeats shocked his family by saying that he remembered "little of childhood but its pain". In fact, he inherited (继承) excellent taste in art from his family — both his father and his brother were painters. But he finally settled on literature, particularly drama and poetry. 

Yeats had strong faith in the coming of new artistic movements. He set himself the fresh task in founding an Irish national theatre in the late 1890s. His early theatrical experiments, however, were not received favorably at the beginning. He didn't lose heart, and finally enjoyed success in his poetical drama. 

Compared with his dramatic works, Yeats's poems attract much admiring notice. The subject matter includes love, nature, history, time and aging. Though Yeats generally relied on very traditional forms, he brought modern sensibility to them. As his literary life progressed, his poetry grew finer and richer, which led him to worldwide recognition. 

He had not enjoyed a major public life since winning the Nobel Prize in 1923. Yet, he continued writing almost to the end of his life. Had Yeats stopped writing at age 40, he would probably now be valued as a minor poet, for there is no other example in literary history of a poet who produces his greatest works between the ages of 50 and 75. After Yeats's Death in 1939, W. H. Auden wrote, among others, the following lines: 

Earth, receive an honoured guest:

        William Yeats is laid to rest. 

        Let the Irish vessel (船) lie

        Emptied of its poetry.

小题1:Which of the following can describe Yeats's family?

A.It filled Yeats's childhood with laughter.

B.It was shocked by Yeats's choice.

C.It was a typically wealthy family.

D.It had an artistic atmosphere.小题2:According to the passage, what do we know about Yeats's life?

A.Yeats founded the first Irish theater.

B.Yeats stuck to modern forms in his poetry.

C.Yeats began to produce his best works from the 1910s.

D.Yeats was not favored by the public until the 1923 Noble Prize.小题3:What kind of feeling is expressed in W. H. Auden's lines?

A.Envy

B.Sympathy

C.Emptiness

D.Admiration小题4:What is the passage mainly about?

A.Yeats's literary achievements

B.Yeats's historical influence

C.Yeats's artistic ambition

D.Yeats's national honor

单项选择题