To taub kev siv ntawm NULLs kom tsis txhob muaj teeb meem Database
Cov neeg siv tshiab rau lub ntiaj teb ntawm databases feem ntau tsis meej pem los ntawm ib qho tshwj xeeb tus nqi tshwj xeeb tshaj tawm rau lub teb - tus nqi NULL. Cov nqi no muaj peev xwm nrhiav tau nyob rau hauv ib cheeb tsam uas muaj hom ntaub ntawv thiab muaj lub ntsiab lus tshwj xeeb heev hauv cov ntsiab lus ntawm cov ntaub ntawv sib paub. Nws yog qhov zoo tshaj plaws los pib peb kev sib tham ntawm NULL nrog ob peb lo lus hais txog dab tsi NUJ tsis yog :
- NULL tsis yog xoom xoom.
- NULL tsis yog tus npliag hlua ("") tus nqi.
Theej, NULL yog tus nqi siv los sawv cev rau ib qho chaw tsis muaj ntaub ntawv. Feem ntau, cov txheej txheem database yuav siv cov kab lus "NULL value", tab sis qhov no tsis yog. Nco ntsoov: ib NUJ yog ib qho tsis muaj nuj nqis uas lub teb tshwm li cas.
NUJ hauv lub Tebchaws tiag tiag
Cia wb mus saib ib qho piv txwv yooj yim: ib lub rooj muaj cov khoom muag rau cov txiv hmab txiv ntoo sawv. Xav tias peb cov khoom muag muaj 10 paj txiv ntoo thiab 3 lub txiv kab ntxwv. Peb kuj stock plums, tab sis peb cov ntaub ntawv cov khoom muag tsis tiav thiab peb tsis paub pes tsawg (yog tias muaj) plums nyob hauv Tshuag. Siv tus nqi NULL, peb yuav muaj cov lus nug cov lus qhia hauv cov lus hauv qab no.
Txiv Ntoo Muag Zaub
| Cov lus nug | Yam khoom | Nqi |
| 1 | Apples | 10 |
| 2 | Txiv kab ntxwv | 3 |
| 3 | Plums | NUJ |
Nws yuav kom meej meej tsis muaj tseeb nrog rau qhov ntau ntawm 0 rau cov ntaub ntawv plums, vim hais tias qhov ntawd yuav hais tias peb tsis muaj plums hauv cov khoom muag. On qhov tsis tooj, peb yuav muaj qee plums, tab sis peb tsis paub tseeb.
TUS NULL los yog tsis thov?
Ib lub rooj ua yuav tsim los yog tso cai rau NULL qhov tseem ceeb los yog tsis.
Ntawm no yog ib qho piv txwv ntawm SQL uas tsim ib qho lus nug uas tso cai rau qee cov NULLs:
SQL> CREATE TABLE INVENTORY (TSWV YIM TSIS TUS NEEG TSIS, TSIM VARCHAR (20) TSIS TXHOB, NQI INT);Cov lus nug lus ntawm no tsis pub LAWV cov nqi rau cov lus nug thiab cov khoom kab, tab sis cia lawv mus rau Kem Sau.
Thaum uas cia tus NUJ tus nqi zoo zoo, LUS qhov tseem ceeb yuav ua rau muaj teeb meem vim tias kev sib piv ntawm qhov tseem ceeb nyob rau hauv ib qho yog NULL ib txwm tau los ntawm NULL.
Txheeb xyuas seb koj cov lus puas muaj nuj nqis npaum li cas, siv qhov YOG NULL los sis TSIS YOG HAIS TIAS neeg teb xov tooj. Ntawm no yog ib qho piv txwv ntawm YOG TUS NEEG:
SQL> SIV INVENTORYID, YAM KHOOM, COV MUAJ CAI LOS NTAWM COV MUAJ CAI NTAWM COV MUAJ CAI ZOO LI TSIS TXHOB;Muab peb tus piv txwv ntawm no, qhov no yuav rov qab los:
| Cov lus nug | Yam khoom | Nqi |
|---|---|---|
| 3 | Plums |
Kev ua haujlwm ntawm NULLs
Ua hauj lwm nrog NULL qhov tseem ceeb feem ntau ua tau HOM tau, nyob ntawm SQL lub lag luam . Piv txwv, piv txwv tias A yog NULL:
Kev Ntsuas Tswv Yim Kho
- A + B = NULL
- A - B = NULL
- A * B = NULL
- A / B = NULL
Sib piv cov neeg teb xov tooj
- A = B = NULL
- A! = B = NULL
- A> B = NULL
- A!
Cov no yog ib co piv txwv ntawm cov neeg tsav tsheb uas yuav rov qab tuaj dua yog tias ib qho lag luam muaj NULL. Ntau txoj kev queries nyob ua ke, thiab tag nrho cov nyuab nyuab los ntawm NULL qhov tseem ceeb. Nqa tus taw tes hauv tsev yog qhov ntawd, yog tias koj tso cai rau NULL qhov tseem ceeb hauv koj cov ntaub ntawv, nkag siab txog qhov teeb meem thiab npaj rau lawv.
Tias 's NULL nyob hauv lub nutshell!