Kumvetsetsa Kugwiritsa Ntchito NULL kuti Pewani Mavuto Achibalo
Ogwiritsira ntchito kudziko la mazambirizidwe nthawi zambiri amasokonezeka ndi mtengo wapadera makamaka kumunda - mtengo wa NULL. Mtengo uwu ukhoza kupezeka mmunda uli ndi mtundu uliwonse wa deta ndipo uli ndi tanthauzo lapadera pakati pa deta yolumikizana. Ndibwino kuti tiyambe kukambirana kwathu kwa NULL ndi mawu ochepa pa zomwe NULL si :
- NULL si nambala ya zero.
- NULL si chingwe chopanda kanthu ("") chofunika.
M'malo mwake, NULL ndi mtengo wogwiritsidwa ntchito kuimira deta yosadziwika. Kawirikawiri, olemba masitepe amagwiritsa ntchito mawu akuti "mtengo wa NULL", koma izi sizolondola. Kumbukirani: NULL ndi mtengo wosadziwika umene munda umawonekera wopanda kanthu.
NULL mu Dziko Leniweni
Tiyeni tiwone chitsanzo chophweka: tebulo yomwe ili ndi mndandanda wa choyimira cha zipatso. Tiyerekeze kuti zolemba zathu zili ndi maapulo 10 ndi malalanje atatu. Timagwiritsanso ntchito ma plums, koma zomwe timaphunzira sizikwanira ndipo sitikudziwa kuti zingati (ngati zilipo) zowonjezera zilipo. Pogwiritsa ntchito mtengo wa NULL, tikhoza kukhala ndi tebulo yosungirako yomwe ikuwonetsedwa mu tebulo ili m'munsiyi.
Chipatso Choyimira Cholowa
| InventoryID | Chinthu | Chiwerengero |
| 1 | Maapulo | 10 |
| 2 | Mawang'anga | 3 |
| 3 | Pewani | NULL |
Zingakhale zolakwika kuti ziphatikize kuchuluka kwa 0 pa zolembera zapums, chifukwa izo zikutanthauza kuti ife sitinakhale ndi zolembera. M'malo mwake, tikhoza kukhala ndi plums, koma sitikudziwa basi.
Ku NULL kapena NOT NULL?
Gome akhoza kupangidwa kuti lilole zikhalidwe za NULL kapena ayi.
Pano pali chitsanzo cha SQL chomwe chimapanga tebulo la Inventory limene limalola ma NULL ena:
SQL> Pangani ZINTHU ZOPHUNZITSIDWA (InventoryID INT NOT NULL, Item VARCHAR (20) OSATI NULL, Chiwerengero INT);Tebulo Lotsalira pano sililola zikhalidwe za NULL za InventoryID ndi zipilala zazitsulo, koma zimawalola kuti zikhale Zowonjezera .
Ngakhale kulola kulemera kwa NULL kuli bwino kwambiri, zikhalidwe za NULL zingayambitse mavuto chifukwa kulinganitsa kwa zikhalidwe zomwe wina ali NULL nthawizonse kumabweretsa NULL.
Kuti muwone ngati tebulo lanu liri ndi malonda a NULL, gwiritsani ntchito NULL kapena SITIRI NER. Pano pali chitsanzo cha NULL:
SQL> SANKHANI INVENTORYID, ITEM, QUANTITY FROM INVENTORY ZIMENE QUANTITY SI NULL;Chifukwa cha chitsanzo chathu apa, izi zikanabwerera:
| InventoryID | Chinthu | Chiwerengero |
|---|---|---|
| 3 | Pewani |
Kugwira ntchito pa NULLs
Kugwira ntchito ndi zikhalidwe za NULL kumabweretsa zotsatira za NULL, malingana ndi ntchito ya SQL . Mwachitsanzo, poganiza kuti A ndi NULL:
Arithmetic Operators
- A + B = NULL
- A - B = NULL
- A * B = NULL
- A / B = NULL
Kuyerekezera Opanga
- A = B = NULL
- A = = B = NULL
- A> B = NULL
- A!
Izi ndi zitsanzo chabe za ogwira ntchito omwe nthawi zonse adzabwezera NULL ngati mmodzi opond ndi NULL. Mafunso ovuta kwambiri, ndipo onse ndi ovuta ndi zikhalidwe za NULL. Mfundo yobwera kunyumba ndi yakuti, ngati muloleza zikhalidwe za NULL mu deta yanu, mvetserani tanthauzo ndi ndondomeko kwa iwo.
Ndilo NULL mwachidule!