Qonda ukusetshenziswa kwe-NULLs ukugwema izinkinga ze-Database
Abasebenzisi abasha emhlabeni jikelele yolwazi bavame ukudideka ngenani elikhethekile ikakhulukazi ensimini - inani le-NULL. Le nzuzo ingatholakala ensimini equkethe noma yiluphi uhlobo lwedatha futhi inencazelo ekhethekile kakhulu kumongo wesitatimende sokuxhumana. Kungcono kakhulu ukuqala ingxoxo yethu ye-NULL ngamagama ambalwa ngalokho uNULL engekho :
- I-NULL ayiyona inombolo yenombolo.
- I-NULL ayiyona inhlamvu engenalutho ("").
Kunalokho, i-NULL iyinani elisetshenziselwa ukumelela idatha engaziwa. Ngokuvamile, abahleli besizinda bazosebenzisa inkulumo ethi "inani le-NULL", kodwa lokhu akulungile. Khumbula: i-NULL iyinani elingaziwa lapho insimu ibonakala ingenalutho.
I-NULL e-World Real
Ake sibone isibonelo esilula: itafula eliqukethe ukufakwa kwesitezi sesithelo. Ake sithi uhlu lwethu luqukethe ama-apula angu-10 nama-oranges amathathu. Siphinde sithenge ama-plums, kodwa imininingwane yethu yokusungula ayiphelele futhi asazi ukuthi zingaki (uma zikhona) amapulazi asesitokisini. Ukusebenzisa inani le-NULL, sizoba nethebula lokubala eliboniswe etafuleni elingezansi.
I-Fruit Stand Inventory
| InventoryID | Into | Inani |
| 1 | Ama-apula | 10 |
| 2 | Ama-oranges | 3 |
| 3 | I-Plums | I-NULL |
Kuzocaca ukuthi akulungile ukuthi kufaka inani elingu-0 lokurekhoda kwe-plums, ngoba lokho kungasho ukuthi sasingenayo i- plums ekusungulweni. Ngokuphambene nalokho, singase sibe nama-plums, kodwa asiqiniseki nje.
Ku-NULL noma CHA NULL?
Ithebula lingakhelwe ukuvumela noma ukuvumela amanani we-NULL noma cha.
Nasi isibonelo se-SQL esakha itafula le-Inventory evumela amanye ama-NULL:
I-SQL> DALA I-TABLE INVENTORY (InventoryID INT NOT NULL, Item VARCHAR (20) AKUKHO NULL, Inani INT);Ithebula le-Inventory lapha alivumeli amanani we-NULL we- InventoryID namakholomu we- Item , kodwa uyawavumela ukuba ikholomu ye- Quantity .
Ngesikhathi uvumela inani le-NULL lihle kakhulu, amanani we-NULL angabangela izinkinga ngoba noma yikuphi ukuqhathaniswa kwamagugu okuyi-NULL njalo kuphumela ku-NULL.
Ukuze uhlole ukuthi ithebula lakho liqukethe amanani we-NULL, sebenzisa i-NULL noma ayiyena umqhubi we-NULL. Nasi isibonelo se-NULL:
I-SQL> Khetha i-INVENTORYID, ITEM, QUANTITY FROM FROM INVENTORY LAPHO I-QUANTITY AYINYE NULL;Sinikezwe isibonelo sethu lapha, lokhu kuzobuya:
| InventoryID | Into | Inani |
|---|---|---|
| 3 | I-Plums |
Isebenza kuma-NULL
Ukusebenza ngamanani we-NULL ngokuvamile kuveza imiphumela ye-NULL, kuye ngokuthi umsebenzi we- SQL . Isibonelo, ucabanga ukuthi i-A yi-NULL:
Abasebenzisi be-Arithmetic
- A + B = NULL
- A - B = NULL
- A * B = NULL
- A / B = NULL
Ukuqhathanisa Abasebenzisi
- A = B = NULL
- A! = B = NULL
- A> B = NULL
- A!
Lezi zibonelo ezimbalwa zezinhlelo zokusebenza ezizobuyela njalo ku- NULL uma u-operand oyedwa engu-NULL. Imibuzo eningi eyinkimbinkimbi ikhona, futhi konke kuyinkimbinkimbi ngamanani we-NULL. Iphuzu lokuthatha ekhaya liwukuthi, uma uvumela amanani we-NULL ku-database yakho, qondanisa impendulo futhi uhlele.
Lokho kuyi- NULL ngokuphazima kweso!