Ib tug superkey yog tus cwj pwm uas siv tau los txheeb xyuas cov ntaub ntawv database
Ib tug superkey yog ib tus los yog ib qho kev sib txuam ntawm cov cwj pwm uas yuav siv tau los txheeb xyuas cov ntaub ntawv teev keeb kwm. Ib lub rooj zaum muaj ntau yam kev sib txuas uas tsim cov duab superkeys.
Superkey Piv txwv
Ib lub rooj nrog lub teb , , thiab , piv txwv li, muaj ntau ntau tau superkeys. Peb superkeys yog , , Lub npe> thiab .
| Lub npe | Hnub nyoog | SSN | Xov tooj Ext. |
|---|
| Robert Jones | 43 | 123-45-6789 | 123 |
| Beth Smith | 43 | 234-56-7890 | 456 |
| Robert Jones | 18 | 345-67-8901 | 789 |
Raws li koj tau pom, txhua kab thiab muaj ntau yam nkag nrog cov lus qhia zoo tib yam. Thaum lub sab siv tau los nrhiav tus neeg, ib tus xov tooj txuas tuaj yeem hloov tau.
Hom kev Superkeys
Ntawm cov uas tau sau tseg hauv cov lus saum toj no, tsuas yog yog tus yuam sij neeg qhib , uas yog ib qho tshwj xeeb ntawm cov neeg ua haujlwm uas siv tsawg tsawg ntawm cov cwj pwm los txheeb xyuas cov ntaub ntawv. Lwm kab muaj cov ntaub ntawv tsis tsim nyog los txheeb xyuas cov ntaub ntawv.
Lub tuaj yeem raug xa mus rau qhov tseem ceeb lossis tsawg tus superkey vim nws muaj qhov tsawg tshaj plaws ntawm cov ntaub ntawv xav tau los txheeb xyuas tus kheej cov ntaub ntawv. Raws li tib cov kab, tus thawj tseem ceeb tseem tuaj yeem yog ib qho superkey thiab ib qhov tseem ceeb vim tias nws yuav tsum tau yoojyim rau cov ntaub ntawv, thiab yuav tsum tsis tshua muaj, yog hloov.
Yog hais tias lub rooj tsis muaj ib tug column ces tus tswv haujlwm yuav tsim cov naj npawb neeg ua haujlwm kom paub txog cov neeg.
Tus lej tshiab cov neeg ua haujlwm yuav raug hu ua tus thawjcoj thawj qhov tseem ceeb. Qhov no surrogate thawj qhov tseem ceeb yuav tau ua ib tus superkey.