Primary Key vs Candidate Key
Txawm tias tus yuam sij tseem ceeb raug xaiv los ntawm tus neeg sib tw tus yuam sij muaj qee qhov sib txawv ntawm tus yuam sij tseem ceeb thiab lwm tus neeg sib tw tus yuam sij, uas yuav tau tham kom ntxaws hauv kab lus no. Kev tsim database yog ib qho tseem ceeb tshaj plaws uas yuav tsum tau ua thaum tswj thiab khaws cov ntaub ntawv. Thaum lub sijhawm tsim cov txheej txheem no, cov rooj sib txawv nrog ntau qhov kev sib raug zoo yuav tsum tau tsim. Txhawm rau nkag mus rau cov ntxhuav no hauv cov ntaub ntawv, cov yuam sij sib txawv tau siv hauv cov ntaub ntawv niaj hnub tsim cov lus xws li MYSQL, MSaccess, SQLite, thiab lwm yam. Tawm ntawm cov yuam sij no, tus neeg sib tw cov yuam sij thiab cov yuam sij tseem ceeb tau dhau los ua qhov tseem ceeb hauv kev tsim qauv database.
Tus Neeg Sib Tw Qhov Tseem Ceeb yog dab tsi?
Tus neeg sib tw tus yuam sij yog ib kem lossis cov kab ke hauv ib lub rooj ntawm cov ntaub ntawv uas tuaj yeem siv los txheeb xyuas cov ntaub ntawv khaws cia yam tsis tau xa mus rau lwm cov ntaub ntawv. Txhua lub rooj ntawm cov ntaub ntawv tuaj yeem muaj ib lossis ntau tshaj ib tus neeg sib tw tus yuam sij. Ib txheej ntawm tus neeg sib tw yuam sij tuaj yeem tsim los ntawm kev siv cov kev ua haujlwm ua haujlwm. Muaj qee qhov tseem ceeb hauv tus neeg sib tw tus yuam sij. Lawv yog;
• tus neeg sib tw tus yuam sij yuav tsum yog qhov tshwj xeeb hauv lub npe thiab lawv yuav tsum tsis muaj qhov muaj nuj nqis NULL.
• tus yuam sij tus neeg sib tw yuav tsum tsis txhob hloov pauv, thiab nws yuav tsum tuav tus nqi tib yam rau qhov tshwm sim ntawm ib qho chaw.
Lub hom phiaj tseem ceeb ntawm tus neeg sib tw tus yuam sij yog los pab txheeb xyuas ib kab ntawm ntau lab kab hauv ib lub rooj loj. Txhua tus neeg sib tw tus yuam sij tsim nyog los ua tus yuam sij tseem ceeb. Txawm li cas los xij, tawm ntawm txhua tus yuam sij tus neeg sib tw, qhov tseem ceeb tshaj plaws thiab tshwj xeeb tus neeg sib tw tus yuam sij yuav dhau los ua tus yuam sij tseem ceeb ntawm lub rooj thiab nws yog qhov zoo tshaj plaws ntawm cov yuam sij sib tw.
Tus Ntsiab Tseem Ceeb yog dab tsi?
Tus yuam sij tseem ceeb yog tus neeg sib tw tus yuam sij zoo tshaj plaws ntawm lub rooj uas yog siv los txheeb xyuas cov ntaub ntawv tshwj xeeb uas khaws cia hauv lub rooj. Thaum tsim ib lub rooj tshiab hauv cov ntaub ntawv peb raug nug kom xaiv tus yuam sij tseem ceeb. Yog li ntawd, xaiv tus yuam sij tseem ceeb rau lub rooj yog qhov kev txiav txim siab tseem ceeb tshaj plaws uas yuav tsum tau coj los ntawm tus tsim database. Qhov kev txwv tseem ceeb tshaj plaws, uas yuav tsum tau txiav txim siab thaum txiav txim siab lub ntsiab tseem ceeb, yog tias cov kab lus xaiv ntawm lub rooj yuav tsum tsuas muaj cov txiaj ntsig tshwj xeeb, thiab nws yuav tsum tsis txhob muaj cov nqi NULL. Qee tus yuam sij tseem ceeb uas feem ntau siv thaum tsim cov rooj yog Tus Naj Npawb Xaus Saus (SSN), ID thiab Daim Npav Npav Npav Tebchaws (NIC).
Tus programmer yuav tsum nco ntsoov xaiv tus yuam sij tseem ceeb ua tib zoo vim nws nyuaj rau hloov. Yog li ntawd, raws li cov programmers, qhov kev coj ua zoo tshaj plaws ntawm kev tsim tus yuam sij tseem ceeb yog siv lub hauv paus tseem ceeb hauv kev tsim tawm xws li Cov Ntaub Ntawv ID tsim los ntawm AutoNumber cov ntaub ntawv hom MS Access. Yog tias peb sim ntxig cov ntaub ntawv rau hauv lub rooj nrog lub ntsiab tseem ceeb uas duplicates cov ntaub ntawv uas twb muaj lawm, qhov kev ntxig yuav ua tsis tiav. Tus nqi tseem ceeb tseem ceeb yuav tsum tsis txhob hloov pauv, yog li nws tseem ceeb dua kom khaws tus yuam sij zoo li qub.
Tus yuam sij tseem ceeb yog tus yuam sij zoo tshaj plaws.
Qhov txawv ntawm Primary Key thiab Candidate Key yog dab tsi?
• Ib tus neeg sib tw tus yuam sij yog kem uas tsim nyog raws li qhov tshwj xeeb whereas thawj tus yuam sij yog kem uas tsis paub txog cov ntaub ntawv.
• Lub rooj uas tsis muaj tus neeg sib tw tus yuam sij tsis sawv cev ib qho kev sib raug zoo.
• Muaj peev xwm muaj ntau tus neeg sib tw tus yuam sij rau lub rooj hauv cov ntaub ntawv, tab sis yuav tsum tsuas muaj ib qho tseem ceeb rau lub rooj.
• Txawm hais tias tus yuam sij tseem ceeb yog ib qho ntawm tus neeg sib tw tus yuam sij, qee zaum nws tsuas yog tus neeg sib tw tus yuam sij xwb.
• Thaum xaiv tus yuam sij tseem ceeb, lwm tus neeg sib tw yuam sij dhau los ua tus yuam sij tshwj xeeb.
• Xyaum tus yuam sij tus neeg sib tw tuaj yeem muaj NULL qhov tseem ceeb txawm hais tias nws tam sim no tsis muaj nqi. Yog li, tus yuam sij tus neeg sib tw tsis tsim nyog rau tus yuam sij tseem ceeb vim tias tus yuam sij tseem ceeb yuav tsum tsis muaj NULL qhov tseem ceeb.
• Nws kuj tseem tuaj yeem ua tau tias tus yuam sij tus neeg sib tw, uas yog qhov tshwj xeeb tam sim no, tuaj yeem muaj cov txiaj ntsig sib npaug uas tsis tsim nyog tus neeg sib tw tus yuam sij los ntawm kev ua tus yuam sij tseem ceeb.
Summary:
Primary Key vs. Candidate Key
Tus neeg sib tw tus yuam sij thiab tus yuam sij tseem ceeb yog cov yuam sij tseem ceeb uas siv los tsim cov ntaub ntawv txhawm rau txhawm rau txheeb xyuas cov ntaub ntawv hauv cov ntaub ntawv thiab ua kev sib raug zoo ntawm cov ntxhuav ntawm cov ntaub ntawv. Ib lub rooj yuav tsum tsuas muaj ib tus yuam sij tseem ceeb thiab yuav muaj ntau dua ib tus yuam sij sib tw. Niaj hnub no, feem ntau cov databases muaj peev xwm pib tsim lawv tus kheej qhov tseem ceeb. Yog li ntawd, tus yuam sij tseem ceeb thiab tus neeg sib tw tus yuam sij muab kev txhawb nqa ntau heev rau kev tswj hwm kev tswj hwm cov ntaub ntawv.
