设 U 是所有属性的集合，X、Y、Z 都是 U 的子集，且 Z=U-X-Y。下列关于多值依赖 的叙述中，______是正确的。 Ⅰ．若 X Y，则 X Y Ⅱ．若 X Y，则 X Y Ⅲ．若 X Y，且Y '' Y ，则 X Ⅳ．若 X Y，则 X Z
A．只有Ⅱ
B．只有Ⅲ
C．Ⅰ和Ⅲ
D．Ⅱ和Ⅳ
参考答案：D
解析：多值依赖具有以下性质：（1）若X→→Y，则X→→Z，所以IV是正确的。（2）若X→Y，则X→→Y，所以II是正确的，I是错误的。（3）设属性集之间的包含关系为：XY包含于W，W包含于U，那么X→→Y在R（U）上成立时，X→→Y也 在R（W）上成立；反过来当X→→Y在R（W）上成立时，X→→Y在R（U）上不一定成立，即多值依赖的有效 性与属性集的范围有关。（4）若 X→→Y，且 Y'包含于 Y，我们不能断言 X→→Y'，所以 III 是错误的。
| 完形填空。 | ||||
| In front of my daughter Sindu, was a bowl filled with cabbage. She hated cabbage. She only sat there in silence with tears 1 her eyes. "Sindu, darling, why don't you 2 a little? Your mom will shout at me." Sindu wiped (擦) her 3 with the back of her hand. "OK, Dad. I'll eat, just a little." Then Sindu said, "Dad, if I eat it all, will you give me 4 I want?" "Yes, within reason." 5 she began to eat. It took her almost half an hour to finish the 6 Then she came up to me and said, "Dad, I want to have my head shaved (刮、剃)!" "Sindu, why don't you 7 something else? If we see you with a clean-shaven head, we will be 8 ," I said. "But, Dad, you promised! Now, you are going back on your words." I had to 9 my promise. Sindu had all her hair cut off. She looked beautiful. On Monday morning, I took her to school. 10 when we arrived, a boy got out of a car, and shouted," Sindu, please wait for me!" Guess what? The boy had 11 hair either. "Sir, your daughter is 12 !" a man got out of the car and said. "That boy is my son, Harish. He has cancer. He lost his hair 13 he got ill. Sindu cut hers off so Harish felt less 14 She's a special girl." I stood for a while, "My little girl, you 15 me how selfless (无私的) real love is!" | ||||
|