Ukusebenza nge database: Kungani izicelo

Uyini "isicelo"? Yini ngokuvamile ukuthini umbuzo database? Yiziphi izicelo ukuthi ithunyelwa database? Ngaphansi isicelo ibeke izimpahla kwezimo ezithile, owawuthi semininingwane ngokuqinisekile ngiyonika impendulo futhi kuhlinzeke imininingwane ngokufinyelela kwezinhloso zenzuzo. okungukuthi khona zithunyelwe ezithile izimo / idatha lapho kunesidingo Imininingwane okungukuthi ezikhethiwe futhi adluliselwa iklayenti ohlangothini / noma sangena database. Impendulo yombuzo "yini into, isicelo", nakanjani nifunde lesi sihloko.

Kungani imibuzo database?

impendulo kumele inikezwe hhayi kuphela embuzweni lokho isicelo, kodwa futhi ku umbuzo, kungani imibuzo. Ziyakwazi ezidingekayo ukuze uthole ulwazi ukuthi lilondolozwe database. Vele baba, database imibuzo eziklanyelwe ulwazi ezidingekayo kwezinye izimo. inhloso yabo zingase zishintshe kakhulu: kungase kudingeke ukuhlonza njengomuntu ikhasimende ebhange kusayithi third-party, noma ukukhomba njengenhlangano intranet umsebenzi, noma ukuthola ulwazi mayelana nesimo se-iphrofayli kwi-website ye umdlalo.

Yiziphi izingxenye embuzweni?

Siyaqhubeka baphendule lo mbuzo: Kungani izicelo. SQL isetshenziswa ukubhala imibuzo. Isidingo sokuthi kube izingxenye ezimbili kuphela: KHETHA futhi FROM. Kodwa ngaphandle kwabo ingasetshenziswa futhi isetshenziswa eziningi nezinye amaqembu ukuthi wengeze izimfuneko ezintsha for Ukukhetha idatha kanye Ukuhlolwa ukubonisa. Lezi zicelo kukhona database ukuthola ulwazi olufanele kakhulu kukhompyutha: udinga ukuthola lapho kuyinto ukubheka. Ethandwa kakhulu ingxenye izingxenye liyadingeka emva kuphi. Lapho kusebenza, ukuze abeke imibandela ethize ukukhethwa idatha. Ngokwesibonelo, ungacacisa inombolo kamazisi, usuku lokuzalwa, noma eminye imininingwane uhlukile futhi engase sikhombe umuntu.

Isakhiwo umbuzo

Noma yisiphi isicelo has yokwehlayo esiqinile ukwakhiwa, okungakwazi aphula. Ukuze nephutha kungenzeka. Ukwakhiwa izochazwa ngesisekelo nombuzo elula ne izingxenye ezintathu. Okokuqala kukhona KHETHA, KUSUKA, futhi kuphi. Opharetha kungenziwa uyithayiphe kokubili izinhlamvu ezinkulu nezincane, ukuba abantu babulawelwe kuzo ayiphazamiseki. Kodwa ngokuvumelana nemithetho yokuziphatha, zonke opharetha zilotshiwe ngohlamvu capital kanye nemibandela edingekayo, amagama amatafula nezinto ne kancane. Futhi ukuze kube lula kakhudlwana ukuba baphumelele kulolu ngenkathi upheqa ikhodi. Ukubuyela ikhodi, kufanele kube eceleni usho lokho kufanele aphendule ngalokho.

Isakhiwo umbuzo, njengoba umthetho, akahlukile emsebenzini e ezahlukene zokuthuthukisa izindawo. Ngakho, ubumi phambi kwakho umbuzo: "Kungani izicelo zokufinyelela» noma omunye imvelo nentuthuko, futhi ungaqiniseka ukuthi izimpendulo ezanikezwa akulesi sihloko kubo bonke beze.

Isicelo idatha Eziyisisekelo

The main ingxenye izingxenye, njengoba kushiwo ngaphambili, ababili kuphela:

• KHETHA [1 into okudingayo 2, udinga 3] - esetshenziswa ucacise ukuthi yiluphi ulwazi oludingekayo. Lokho-ke uzobe adluliselwe semininingwane uhlelo ngalo umsebenzisi isebenza.
• KUSUKELA [ithebula kusuka iyiphi idatha zithathwa] - Cacisa edingekayo idatha okungukuthi encane, futhi namanje isidingo akhombise lapho kufanele kuthathwe. I database is hhayi egcinwe ngqo idatha, kanye itafula ukuthi has idatha. Lamatafula ahlukahlukene angaba amakholomu efanayo idatha ukugwema lokhu, futhi isetshenziselwa ucacise lapho esithathwayo.

Imininingwane engeziwe kanye nokusebenza isicelo iqembu

Ukuze ukuthuthukisa imiphumela yosesho kanye nokunikeza ulwazi mayelana nokusetshenziswa kolwazi esenizitholile imiyalo eyengeziwe:

• Lapho [osesho] - esezisetshenzisiwe lungisa adingekayo Imininingwane ngokuqondene othize yokufaneleka izimo.
• -LIMIT [inombolo] - isetshenziselwa zinciphise isibalo imigqa ukuthi uyothathwa etafuleni.
• GROUP BY [lombuzo wepharamitha] - isetshenziswa iqembu le wathola ukwaziswa okuvela database. Kodwa iqembu ayikhona Imininingwane kungenziwa enikeziwe, kodwa kuphela ngokulingana, nokuba yohlobo olufanayo. Ukuze uthole imininingwane eyengeziwe ungakwazi ukuthola ngokuthola isihloko ehlukene kwi GROUP BY. imisebenzi yeqembu imibuzo eklanyelwe ukuthuthukisa ukubukeka ulwazi kanye readability kwaso okukhulu.
• INYUNYANA [isicelo] isetshenziswa ukubeka ehlukile isicelo subquery. Uma uthola kungenzeka inani sasivumela abalulekile kolwazi kudingeke nokuthi kukho.
• NJENGO "" isetshenziswa isheke ukuthi imaskhi oluhambisana isicelo othile usayizi idatha. Ngakho, nge zayo usizo, Unga cinga umuntu i iholo vymeryat eziyisithupha izibalo.

Ngokwesibonelo

Lokhu kukodwa kuyisizathu ukuqonda eziyinkinga labhaliwe ngaphandle isibonelo okuhambisanayo. Kodwa ngisho olulodwa Ngokwesibonelo abakwazi ukuchaza konke, futhi une ukuthola ulwazi oluningi ngaphambi uzokwazi bamba yonke amathuba ahlinzekwa SQL Onjiniyela:

KHETHA Igama, ProductNumber, ListPrice NJENGOBA Inani

KUSUKELA Production.Product

LAPHO ProductLine = 'R'

Lokuhlwaya langibonisa ikhodi engenhla. Okokuqala, kukhona ukukhethwa idatha ezidingekayo: igama, inombolo umkhiqizo futhi amanani ikhasi Ipheshana. Futhi ishidi intengo siboniswa igama ukuhluka - a "intengo". Idatha ethathwa semininingwane "Imikhiqizo" etafuleni "Mkhiqizo". Ngokuvamile, ucacise semininingwane okungukuthi akudingekile uma usebenza kuphela eyodwa, empeleni Ithumela isicelo. Kodwa uma izisekelo ambalwa, bese qiniseka ucacise, unikezwa iqiniso ukuthi kukhompyutha nje ngeke baqonde kuye ubheke noma bangawanaki sesicelo sakho kanye nokubalulwa ezokwenzela nephutha. Umugqa wesithathu ibonisa ukuthi okukhiphayo akuyona yonke imininingwane, kodwa lowo eliphuma khona umuda "P" imikhiqizo. Ngagcina lapho isihloko esifushane, ngemva Ukufundwa ngalo manje baqonde ukuthi kukhona imibuzo.