问题
问答题
设有关系模式R(A，B，C，D)，在其上的函数依赖集为：
F=A→C，B→AC，D→AC，C→A)
①计算(CD)+。
②求R的码。
答案
参考答案：①(CD)+=ACD
②B和D在F中所有函数依赖中均未在右部出现，候选码中一定存在BD。而(BD)+=ABCD，所以BD是R的码。
搜题请搜索小程序“爱下问搜题”
设有关系模式R(A，B，C，D)，在其上的函数依赖集为：
F=A→C，B→AC，D→AC，C→A)
①计算(CD)+。
②求R的码。
参考答案：①(CD)+=ACD
②B和D在F中所有函数依赖中均未在右部出现，候选码中一定存在BD。而(BD)+=ABCD，所以BD是R的码。
| 阅读理解。 | ||||
| ||||
| 1. Which of the following books is the latest in print? | ||||
[ ] | ||||
| A. Harry Potter and the Deathly Hallows. B. Deceptively Delicious. C. Rich Dad, Poor Dad. D. The Road. | ||||
| 2. Among the books, there is / are written for adults. | ||||
[ ] | ||||
| A. one B. two C. three D. four | ||||
| 3. Who wrote Rich Dad, Poor Dad? | ||||
[ ] | ||||
| A. J.K. Rowling. B. Jessica Seinfeld. C. Cormac McCarthy. D. Robert T. Kiyosaki. | ||||
| 4. If you buy two copies of The Road and one copy of Deceptively Delicious, you should pay ______. | ||||
[ ] | ||||
| A. $23.83 B. $18.33 C. $12.83 D. $11 | ||||
| 5. We can learned from the form (图表) that . | ||||
[ ] | ||||
| A. Deceptively Delicious and The Road came out on the same day B. J.K Rowling's book is the most expensive among the four books C. The Road is the thinnest of the four D. Rich Dad, Poor Dad is hardcover |
