Qonda ukusetyenziswa kwe-NULLs ukukhusela iingxaki ze-database
Abasebenzisi abatsha kwihlabathi yolwazi bahlala bedideka ngokubaluleka okhethekileyo kwintsimi-inani le-NULL. Eli xabiso linokufumaneka kwintsimi equkethe nayiphi na idatha yedatha kwaye inentsingiselo ekhethekileyo kumxholo wesiseko sedatha. Kungcono kakhulu ukuqala ingxoxo yethu ye-NULL ngamagama ambalwa malunga ne-NULL engekho :
- I-NULL ayiyona inombolo yero.
- I-NULL ayilona intambo engenanto ("").
Kunoko, i-NULL yixabiso elisebenzisayo ukumela into engaziwayo yedatha. Ngokuqhelekileyo, abacwangcisi beenkcukacha ziza kusetyenziswa ibinzana elithi "inani le-NULL", kodwa oku kulungile. Khumbula: i-NULL yixabiso elingaziwa apho intsimi ibonakala ingenanto.
I-NULL kwiLizwe leNene
Makhe sibone umzekelo olula: itafile equkethe isistim sokuma kwesityalo. Masithi ukuba uluhlu lwethu luqulethe ii-apula ezili-10 kunye nama-oranges amathathu. Siphinde sisebenzise i-plums, kodwa ulwazi lwethu lolwazi aluphelelanga kwaye asazi ukuba zininzi (ukuba zikhona) iiplums zikhoyo. Ukusebenzisa ixabiso le-NULL, siza kuba netheyibhile yolwazi eboniswe kwitheyibhile elingezantsi.
Imveliso yokuHlalwa kweZithelo
| InventoryID | Into | Ubungakanani |
| 1 | IiApile | 10 |
| 2 | Oranges | 3 |
| 3 | Plums | NULL |
Kuya kubonakala ukuba ayilunganga ukufaka ubungakanani be-0 kwiirekhodi ze-plums, kuba oko bekuya kuthetha ukuba sasingenayo i- plums kwi-inventory. Ngokuphambene noko, sinokuba neepum, kodwa asiqinisekanga.
Ku-NULL okanye NGA-NULL?
Itafile zingenzelwe ukuba zivumele ixabiso le-NULL okanye cha.
Nantsi umzekelo we-SQL odala itafile ye-Inventory evumela ezinye i-NULL:
I-SQL> YOKHALA ITHEBULI YOKUPHAKATHI (InventoryID INT NOT NULL, Item VARCHAR (20) AKAZI NULL, Ubungakanani INT);Itheyibhile yokuThengisa apha ayivumeli ixabiso le-NULL kwi- InventoryID kunye nekholamu ye- Item , kodwa iyabavumela ukuba kukho ikholamu yexabiso .
Nangona ukuvumela ixabiso le-NULL lihle kakuhle, ixabiso le-NULL linokubangela iingxaki kuba kukho nawuphi na ukuthelekiswa kwexabiso kwi-NULL ihlala iphumela kwi-NULL.
Ukujonga ukuba itheyibhile yakho iqulethe ixabiso le-NULL, sebenzisa i-NULL okanye ayiyi-operator ye-NULL. Nasi umzekelo we-NULL:
I-SQL> UKUKHETHA INVENTORYID, ITEM, QUANTITY FROM FROM INVENTORY WHERE QUANTITY AYI-NULL;Njengoko umzekelo wethu apha, oku kuza kubuya:
| InventoryID | Into | Ubungakanani |
|---|---|---|
| 3 | Plums |
Ukusebenza kwi-NULLs
Ukusebenza ngeempawu ze-NULL kudla ngokuvelisa iziphumo ze-NULL, kuxhomekeke kwi- SQL . Umzekelo, ucinga ukuba i-A yi-NULL:
AbaSebenzi beArithmetic
- A + B = NULL
- A - B = NULL
- A * B = NULL
- A / B = NULL
Ukuqhathanisa nabasebenzisi
- A = B = NULL
- A = = B = NULL
- A> B = NULL
- A!
Le yimizekelo nje yabasebenzisi abaza kuhlala bebuyela i-NULL ukuba omnye u-operand ngu-NULL. Imibuzo enzima kakhulu ikhona, kwaye zonke ziyinkimbinkimbi ngamaxabiso e-NULL. Ukuthabathela kwinqanaba lasekhaya kukuba, ukuba uvumela ixabiso le-NULL kwisiseko sakho seenkcukacha, qondanisa impembelelo kunye nesicwangciso kubo.
Yiyo i- NULL ngokucacileyo!