问题 填空题

不改变关系表中的属性个数但能减少元组个数的是______。

答案

参考答案:选择

解析: 关系数据库的专门关系运算包括选择、投影和连接3种,其中选择不改变关系表中的属性个数但能减少元组个数。

选择题
[ ]

A. supermarket              

B. park                        

C. library

3. When do we use umbrella and boots? It's _____.     

A. rainy                    

B. sunny                      

C. windy

4. It's a thing. When you go on a trip and take some pictures, you will take it. It's a _____.     [ ]

A. tent                      

B. umbrella                    

C. camera

5. What has hands but no feet, a face but no eyes, tells but does not talk? It's a _____ .           [ ]

A. table                    

B. blackboard                  

C. clock

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