问题 单项选择题

如果对一个关系实施了一种关系运算后得到了一个新的关系,而且新的关系中属性个数少于原来关系中属性个数,这说明所实施的运算关系是 ( )

A.选择

B.投影

C.连接

D.并

答案

参考答案:B

解析: 选择运算是对二维表进行行切割,执行后会删除某些元组。投影运算是对二维表的列操作,执行后去除某列属性。连接运算有选择的将两关系中取出的元组组合,属性列并不会减少。并运算同样不会减少属性列的个数。根据以上知识点可知选项B正确。

阅读理解

Greeks play a special game at Easter and it’s good fun! The rules of the game are as follows: 1.Everyone picks out a hard-boiled Waster Egg.2.Each player finds a partner.One person will be a “holder”, and the other will be a “tapper”.Holders should hold the egg with either the pointy end or the round end facing up.Tappers should hold the egg with that same end facing down.Tapping must be done pointy-end-to-pointy-end or round-end-to-round-end.3.With one good tap, the tapper tries to crack the holder’s egg.One player will end up with a cracked egg.4.Each player finds another partner and repeats steps 2 and 3.(After one end of the egg is cracked, players may continue to play using the uncracked end).5.A player loses when both ends of his egg have been cracked.The winner is the person who still has an uncracked egg.6.Enjoy eating the eggs!

Dye your Easter eggs.With an adult’s help, dye hard-boiled eggs red using food coloring or egg dye.For a nice sheen, use a paper towel to rub some oil on each egg.But don’t use too much oil---you wouldn’t want your egg to slide out of your hand when you play the egg-cracking game!

Why dye eggs red? Eggs represent new life and the red dye symbolizes the pain and suffering in the past.

Why crack eggs? Some say that the cracking of the eggs symbolizes a wish to break away from human misery and enter the new life.

小题1:A player has to drop out of the game when _____.

A.the pointy end of his egg is cracked

B.the round end of his egg is cracked

C.his egg slides out of the hand

D.his egg is cracked at both ends小题2:Why is oil rubbed on the egg?

A.To give the egg a shiny appearance

B.To make the game more difficult to play

C.To protect the hand of the player

D.To symbolize the wealth of the Greeks小题3:Which of the following is TRUE according to the passage?

A.The holders hold the eggs with the pointy end facing up in most cases

B.If one end of the player’s egg is cracked, he will eat the egg

C.The player in the game must dye his egg red independently

D.It’s said that the broken eggs represent the wish to lead a new life小题4:Which of the following is the best title for the passage?

A.The suffering of the Greeks

B.The Easter holidays

C.Celebrate Greek Easter

D.A special game

单项选择题