【单选题】【消耗次数：1】
以下叙述中正确的是( )。
①
连接操作中不可以用别名指定数据的来源
②
如果缺省WHERE子句，则会产生错误信息
③
在SQL语言中绝大多数连接操作都是自然连接
④
为了实现连接运算，SELECT命令中必须指出属性的来源
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相关题目
【多选题】
下面关于自然连接和等值连接的叙述中，正确的是（ ）
①
自然连接是一种特殊的等值连接
②
自然连接要求在两个关系中有公共属性，而等值连接不必
③
两种连接都可以只用笛卡尔积和选择运算导出
④
自然连接要在结果中去掉重复的属性，而等值连接不必
【多选题】
下面关于自然连接和等值连接的叙述中，正确的是（ ）
①
A.自然连接是一种特殊的等值连接
②
B.自然连接要求在两个关系中有公共属性，而等值连接不必
③
C.两种连接都可以只用笛卡尔积和选择运算导出
④
D.自然连接要在结果中去掉重复的属性，而等值连接不必
【多选题】
连接操作是查询处理中最常见也是最耗时的操作之一。数据库管理系统中实现连接操作的算法包括（ ）。
①
嵌套循环算法
②
Hash Join算法
③
排序-合并算法
④
索引连接算法.
随机题目
【单选题】
6. –Look, someone is coming. Guess who it _____ be?–I think it _____ be Tom.–I don’tthink it _____ be _____.
①
must; can; must; his
②
may; can; must; him
③
must; can; him
④
might; must; can; himself
【单选题】
1. If they ______ to do this work, he might do it some other way.
①
were
②
should
③
will
④
can
【单选题】
8. I______ go to bed until I ______ finished my work last night.
①
Don’t; had
②
didn’t have
③
didn’t; had
④
don’t; have
【单选题】
7. If it is fine tomorrow, we ______ a football match.
①
have
②
will have
③
has
④
shall has
【单选题】
19. _______hand in his exercise book before Monday.
①
Need he to
②
Does he need
③
Needs he to
④
Need he
【单选题】
9. No sooner _______ he arrived home than he _______to start on another journey.
①
has; was asked
②
have; were asked
③
had; is asked
④
had; was asked
【单选题】
13. Where is my pen? I ______ it.
①
might lose
②
would have lost
③
should have lost
④
must have lost
【单选题】
14. I didn’thear the phone. I ______ asleep.
①
must be
②
must have been
③
should be
④
should have been