Xav txog tus cwj pwm yog ib yam ntxwv
Ib lub database muaj hwj chim tshaj dua li daim ntawv khom nqi rau nws vim nws muaj peev xwm tshawb nrhiav ntau. Cov kab txuas sib txuas ntawm cov ntaub ntawv sib txuas sib txuas hauv cov ntxhuav sib txawv thiab ua cov lus teb rau ntau cov ntaub ntawv sib txuas. Cov ntaub ntawv raug txhim kho nyob rau hauv ib txoj kev uas tswj tau yooj yim, nkag mus, thiab kho tshiab.
Dab Tsi Yog Dab Tsi?
Lub database muaj cov ntxhuav. Txhua lub rooj muaj kab thiab kab.
Txhua kab (hu ua tuple) yog cov lus qhia uas siv rau ib yam khoom. Txhua kem (tus cwj pwm) muaj piav txog tus yam ntxwv ntawm cov kab. Ib tug database attribute yog ib lub npe ntawm lub npe thiab cov ntsiab lus ntawm lub teb raws li nws nyob rau hauv ib lub rooj hauv ib lub database.
Yog tias koj muag cov khoom thiab nkag mus rau hauv ib lub rooj nrog txhua kab rau ProductName, Nqe, thiab ProductID, txhua lub hau lus yog tus cwj pwm. Nyob hauv txhua daim teb raws li cov hau lus, koj nkag mus rau cov khoom npe, cov nqi, thiab cov khoom pov thawj khoom, raws li. Ib qho ntawm kev nkag teb chaws yog ib qho tseem ceeb.
Qhov no yuav ua rau kev txiav txim siab thaum koj xav txog nws, muab tias qhov kev txhais ntawm lub ntsiab lus ntawm tus cwj pwm yog tias nws txhais tau tias yog tus cwj pwm los sis zoo ntawm ib yam dab tsi.
Tus Cwj Pwm Piav Qhia Cov Seem Muaj
Cia peb xav txog lub database uas tsim los ntawm lub lag luam. Nws yuav muaj cov ntxhuav-kuj hu ua cov koom haum los ntawm database designers-rau Cov Neeg Siv khoom, Cov Neeg Ua Hauj Lwm, thiab Cov Khoom, ntawm lwm cov. Cov khoom rooj qhia txog cov yam ntxwv ntawm txhua yam khoom.
Cov no kuj muaj xws li cov khoom ID, lub npe khoom, tus ID ID (siv raws li qhov tseem ceeb txawv teb chaws ), ntau npaum li cas, thiab nqi. Txhua yam ntawm cov yeeb yam no yog tus cwj pwm ntawm lub rooj (los yog qhov chaw) npe khoom.
Xav seb qhov snippet no ntawm cov ntaub ntawv feem ntau-raug nplua Northwinds:
| ProductID | ProductName | Muab khoom siv | Qeb | QuantityPerU | UnitPrice |
|---|---|---|---|---|---|
| 1 | Chai | 1 | 1 | 10 lub thawv x 20 hnab | 18.00 |
| 2 | Tseeb | 1 | 1 | 24 - 12 oz fwj | 19.00 |
| 3 | Aniseed Syrup | 1 | 2 | 12 - 550 ml hwj | 10.00 |
| 4 | Tus kws kho mob Anton tus Cajun Seasoning | 2 | 2 | 48 - 6 oz txhiab | 22.00 |
| 5 | Tus kws lij choj Anton lub Gumbo Mix | 2 | 2 | 36 lub thawv | 21.35 |
| 6 | Pog yawg Grand Boysenberry kis mus | 3 | 2 | 12 - 8 txhiab txhiab | 25.00 |
| 7 | Txiv ntxawm Bob lub Organic qhuav Pears | 3 | 7 | 12 - 1 lb pkgs. | 30.00 |
Kem npe yog cov cwj pwm ntawm cov khoom. Cov ntawv sau rau ntawm cov kab txhua tus kuj tseem ceeb ntawm cov khoom.
Puas yog Tus Qauv Coj?
Qee lub sij hawm, lub sij hawm teb thiab tus cwj pwm yog siv kev sib hloov, thiab rau ntau lub hom phiaj, lawv yog tib yam nkaus. Txawm li cas los xij, feem ntau yog siv los piav txog ib lub xov tooj ntawm tes nyob rau hauv ib lub rooj nyob ntawm txhua tus kab, thaum uas tus cwj pwm yog feem ntau yog siv los piav qhia lub koom haum tus xeeb ceem nyob rau hauv qhov kev txiav txim siab.
Piv txwv, hauv cov lus saum toj no, ProductName nyob rau sab thib ob yog Tseeb . Qhov no yog ib daim teb . Yog tias koj sib tham txog cov khoom, Feem ntau, ProductName yog qhov khoom ntawm sab. Qhov no yog tus cwj pwm .
Tsis txhob khwb ntawm qhov no. Feem ntau, ob nqe lus no yog siv sib hloov.
Txhais cov Qauv
Cov cwj pwm yog txhais raws li lawv cov npe . Ib tug sau txhais cov kev tso cai uas pom tias qhov yeeb yam no muaj peev xwm muaj. Qhov no yuav muaj nws cov hom ntaub ntawv, qhov ntev, qhov tseem ceeb, thiab lwm yam ntsiab lus.
Piv txwv, tus sau rau tus attribute ProductID yuav qhia meej txog hom ntaub ntawv numeric. Tus cwj pwm yuav tsum tau txhais ntxiv kom xav tau ib lub sijhawm ntev lossis qhia kom meej seb puas muaj nuj nqi npliag los yog tsis paub.