今天考,这是题目和我的答案!

题目:

一.单项选择

1.

Withrespecttoarelationaltable,whatisakey?

(a)Aminimalsubsetofcolumnsthatuniquelyidentifiesarowinthetable (b)TheIDandpasswordneededtoaccessthetable (c)Thesubsetofalltherowsandcolumnsinthetablethatarevisibletoallusersinthedatabasesystem (d)Theonecolumnthatuniquelyidentifiesarowinthetable

2.

Whichofthefollowingistrueaboutthenumberofprimarykeysandalternatekeyswithrespecttoatable?

(a)Therecanbemanyprimarykeys,butonlyonealternatekey. (b)Therecanbeonlyoneprimarykey,butmanyalternatekeys. (c)Therecanbeonlyoneprimarykeyandonlyonealternatekey. (d)Therecanbemanyprimarykeysandmanyalternatekeys.

3.

Aninsertionoperationwill_____iftheinsertionviolatestheuniquenesspropertyofakey.

(a)succeedwithwarning (b)crashthesystem (c)fail (d)succeedwithoutwarning

4.

Aninsertionoperationwill_____iftheinsertedprimarykeyhasaNULLvalue.

(a)succeedwithwarning (b)succeedwithoutwarning (c)fail (d)crashthesystem

5.

Fortwotablestobeunioncompatible,correspondingcolumnsfromeachtableshouldhavewhichofthefollowing?

(a)differentdomains (b)differentnames (c)thesamedomain (d)thesamename

6.

Intherelationalmodel,whichofthefollowingistrueaboutthedatatypeofacolumn?

(a)Itneednotbeatomicanditcannotbeanabstractdatatype. (b)Itmustbeatomicanditcannotbeanabstractdatatype. (c)Itneednotbeatomicanditcanbeanabstractdatatype. (d)Itmustbeatomicanditcanbeanabstractdatatype.

7.

Whatisanalternatekey?

(a)Anykeythatisnotaprimarykey (b)Thekeytousewhentheprimarykeydoesnotwork (c)Akeythatwasaddedtothetableafterthetablewasdesigned (d)Akeythatwillbecometheprimarykeywhentheprimarykeyisdeleted

8.

Whatdoesaprojectionoperationdo?

(a)Itextendsthenumberofcolumnsinatable. (b)Itselectsrowsfromatable. (c)Itextendsthenumberofrowsinatable. (d)Itselectscolumnsfromatable.

9.

Theresultofasetdifferenceoperationr-swillbe

(a)tuplesinrafterdeductingtheirvaluesbythecorrespondingvaluesinthetuplesins (b)tuplesinsafterdeductingtheirvaluesbythecorrespondingvaluesinthetuplesinr (c)thosetuplesthatareinsbutnotinr (d)thosetuplesthatareinrbutnotins

10.

WhichofthefollowingistrueaboutprimarykeysandforeignkeysholdingNULLvalue?1