皮皮学，免费搜题
登录
搜题
【单选题】
设关系R和S的分别为r和s。那么，由属于R但不属于S的元组组成的集合运算称为__（1）__。在一个关系中找出所有满足某个条件的元组的运算称为__（2）__运算。对R和S进行__（3）__运算可得到一个r+s元的元组集合，其每个元组的前r个分量来自R的一个元组，后s个分量来自S的一个元组，如果R中有m个元组，S中有n个元组，则它们经__（4）__运算后共有__（5）__个元组。关系R和S的自然连接运算一般只用于R和S有公共__（6）__的情况。空白（6）处（）A.
元组
B.
属性
C.
关键码
D.
关系模式
手机使用
分享
复制链接
新浪微博
分享QQ
微信扫一扫
反馈参考答案：
举一反三
【简答题】Para.4 Abraham Lincoln 5. What enabled Lincoln to succeed despite all the misfortunes in his life? (Paras. 4) His ___________________.
【简答题】What enabled the earliest balloons to fly? [A] Gas produced by metal and coal. . [B] Air kept hot by burning coal. [C] Hydrogen heated by a. gas burner. [D] Natural gas carried in a metal container.
【单选题】What has enabled the volunteers to rescue all the remaining whales on the beach?
A.
The tide rising at noon.
B.
Buckets of water.
C.
Feverish rescue efforts.
D.
Many dump trucks.
【单选题】What enabled Abraham Lincoln to succeed?
A.
fortune
B.
optimism
C.
efforts
D.
ambition
【单选题】Protocol was ________ enabled him to make difficult decisions without ever looking back.
A.
who
B.
what
C.
which
D.
that
【单选题】You can find _________ you need at the shopping centre, which isalways busy at the weekend.
A.
whichever
B.
wherever
C.
whoever
D.
whatever
相关题目：
参考解析：
知识点：
题目纠错 0
发布
