问题
单项选择题
下面不正确的说法是
A．关键字是关系中能够用来惟一标识元组的属性
B．在一个关系中，关键字的值不能为空
C．一个关系中的所有候选关键字均可以被指定为主关键字
D．关键字只能由单个的属性组成
答案
参考答案：D
解析： 在关系的诸属性中，能够用来惟一表示元组的属性(或属性组合)称为关键字或码。因此，关键字既可以是单个属性，也可以是属性组。
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下面不正确的说法是
A．关键字是关系中能够用来惟一标识元组的属性
B．在一个关系中，关键字的值不能为空
C．一个关系中的所有候选关键字均可以被指定为主关键字
D．关键字只能由单个的属性组成
参考答案：D
解析： 在关系的诸属性中，能够用来惟一表示元组的属性(或属性组合)称为关键字或码。因此，关键字既可以是单个属性，也可以是属性组。
Mr. Black loved his daughter Helen very much. Every day he spent all of his free time 36 with her. One night, when he 37 his house, Helen came running to him and was ready to play. But he had a lot of work to do, 38 couldn’t play with his daughter. In order to keep her 39 , he found a map of the world in a magazine and cut it into pieces. Mr. Black said he had 40 time to play right now. He told the girl to put the pieces of paper back together to make up a map of the world, and then they could both play. About half an hour later, Helen came to her father and said, “Okay, it’s finished. Can we play now?” Mr. Black was surprised, he said, “That’s 41 . Let’s go and see.” And sure enough, there was the picture of the world; all put together, every 42 in its place. Mr. Black said, “That’s surprising! 43 did you do that?” Helen said, “It was 44 . On the back of the paper was a picture of a 45 .When I put the man together, the whole world fell into place.”
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