Sebenzisa i-SQL COUNT ukuze ubuyekeze uhlu olubanzi lwemininingwane
Isici semibuzo yingxenye ebalulekile yolimi olufundwayo olwenziwe (SQL). Ithola idatha ngokusekelwe ezinkambisweni ezithile ezivela kudatha lokuxhumana. Ungasebenzisa imibuzo ye-SQL - kufaka phakathi umsebenzi we-COUNT () - ukuthola zonke izinhlobo zolwazi kusuka ku-database.
Umsebenzi we-SQL COUNT () uwusizo ikakhulukazi ngoba ikuvumela ukuthi ubale amarekhodi egciniwe esekelwe kwimigomo ecacisiwe yomsebenzisi. Ungayisebenzisa ukuze ubale wonke amarekhodi etafuleni, ubale amanani ayingqayizivele kukholomu, noma ubale inani lamarekhodi ambalwa okwenzekayo okuhlangabezana nemigomo ethile.
Lesi sihloko sibheka ngeso lengqondo ngayinye yalezi zimo.
Izibonelo zisekelwe ku-database yaseNorthwind evame ukusetshenziswa, evame ukuthumela ngemikhiqizo yedatha ukuze isetshenziswe njengesifundo.
Nasi i-excerpt etafuleni lomkhiqizo wedatha:
| ProductID | ProductName | I-SupplierID | OkungenaniPerUnit | Intengo yokukodwa | UnitsInStock |
|---|---|---|---|---|---|
| 1 | Chai | 1 | Amabhokisi 10 x izikhwama ezingu-20 | 18.00 | 39 |
| 2 | Chang | 1 | Amabhodlela angu-24 - 12 oz | 19.00 | 17 |
| 3 | Isiraphu esisaziwayo | 1 | Amabhodlela ama-12 - 550 ml | 10.00 | 13 |
| 4 | I-Chef Anton's Cajun Seasoning | 2 | 48 - 6 oz izimbiza | 22.00 | 53 |
| 5 | I-Gumbo Mix Mix ye-Chef Anton | 2 | 36 amabhokisi | 21.35 | 0 |
| 6 | Ugogo we-Boysenberry Spread | 3 | 12 - 8 oz izimbiza | 25.00 | 120 |
| 7 | Amapheya omile omalume ka-Uncle Bob's | 3 | 12 - 1 lb ama-pkgs. | 30.00 | 15 |
Ukubala Amarekhodi Ethebula
Umbuzo oyisisekelo kunazo zonke ubala inani lamarekhodi etafuleni. Uma ufuna ukwazi inani lezinto ezikhona kuthebula lomkhiqizo, sebenzisa umbuzo olandelayo:
HLOLA COUNT (*)
KUSUKA umkhiqizo;
Lo mbuzo ubuyisa inani lemigqa etafuleni. Kulesi sibonelo, ku-7.
Ukubala Amanani Ayingqayizivele Kukholomu
Ungasebenzisa futhi umsebenzi we-COUNT ukukhomba inani lamanani ayingqayizivele kukholomu. Esikhathini, uma ufuna ukuthola inombolo yabahlinzeki abahlukene abavela emikhiqizweni yabo yokukhiqiza, ungayifeza lokhu usebenzisa umbuzo olandelayo:
Hlunga COUNT (DISTINCT SupplierID)
KUSUKA umkhiqizo;
Lo mbuzo ubuyisela inani lamanani ahlukile athola ikholomu ye-SupplierID. Kulesi simo, impendulo ingu-3, emele 1, 2, no-3.
Ukubala Amarekhodi Ukulinganisa Ukulinganisa
Hlanganisa umsebenzi we-COUNT () ne-WHERE clause ukukhomba inani lamarekhodi afana nemigomo ethile. Ngokwesibonelo, ake sithi umphathi wezemfundo ufuna ukuthola umuzwa wamanqanaba esitokisi emnyangweni. Umbuzo olandelayo ubonisa inani lemigqa emele i-UnitsInStock engaphansi kwama-unit angama-50:
HLOLA COUNT (*)
KUSUKA umkhiqizo
LAPHO UnitsIntock <50;
Kulesi simo, umbuzo wawuzobuyisela inani le-4, elimelela i-Chai, i-Chang, i-Syrup Aniseed, ne-Uncle Bob's Organic Pears Pears.
Isigaba somthetho esingu-COUNT () singabaluleka kakhulu kubaqondisi besizinda abafuna ukufingqa idatha ukuze bahlangabezane nezidingo zebhizinisi. Ngokwemvelo encane, ungasebenzisa umsebenzi we-COUNT () ngezinhloso ezihlukahlukene.