通过 (1) 关系运算,可以从表 1 和表 2 获得表 3;表 3 的主键为 (2) 。
1()
A.投影
B.选择
C.笛卡尔积
D.自然连接
参考答案:D
选择与画线部分意思相同或相近的选项。
1. Today is Tuesday. I have lots of homework to do. I'm really busy.
A. not free
B. tired
C. strict
D. relaxed
2. Tom has lunch al 12:00 with his classmates at school.
A. eat
B. have
C. takes
D. eats
3. My father works from Monday to Friday.
A. on weekdays
B. on Friday
C. on Saturday
D. in the day
4. We don't have any classes on weekends.
A. on Wednesday
B. on Monday
C. on Saturday and Sunday
D. on weekdays
5. Mother's Day is in the fifth month of the year.
A. March
B. April
C. September
D. May
简述现代物流的物流信息的特征。