Njengoko ebekwe yi-SQL-ukuhlela?

Xa sisebenza kunye zolwazi qho kuyimfuneko baphethe isiphumo umbuzo ngendlela ethile, umzekelo alfabhethi. Ukuze benze oku, i-DBMS la engenamsebenzi mnye kwi SQL - Ukuhlela. Kulo mzekelo, uhlelo ungakhetha eyiphi imihlaba kwaye apho ukuze kuya kuba ukufezekisa iziphumo ezinqwenelekayo ngaphandle kokusebenzisa izakhono iinkqubo kakhulu.

Yintoni ezibekwe ngayo database?

Ukusebenza edatha oko rhoqo linxibelelene umthamo omkhulu ulwazi, oyimfuneko ukumisela umyalelo. Okwangoku, kukho eziliqela iinkcukacha, nto leyo eye uluhlu olubanzi lweempawu, eyona ethandwa kuzo Oracle ne MS SQL. ulwazi Uhlobo njengenye iinkqubo eziphambili kwi-database, babonelelwa esikhethekileyo eyakhelwe-ngaphakathi ngamnye kubo.

olusuka lula le nkqubo yokukhangela, kwaye kwezinye iimeko, kunceda ukusombulula iingxaki ezithile okanye nokwandisa ukusebenza kwenkqubo. SQL-yokuhluza nangokwenkalo ngokwahlukeneyo ezikhethiweyo, yaye ukuba kuyimfuneko, kwimeko imilinganiselo efanayo nezinto yalo mhlaba unako ukubonisa izicwangciso ezongezelelweyo, leyo iza kucacisa indawo imitya.

uhlobo umyalelo

SQL-yokulungisa kule database inikwa ngokusebenzisa UMYALELO BY kwemisebenzi. Ngoko ke, lo Ukuveliswa yokuba ingcaciso evela kwiziko ledatha, emva kokuba ukhankanye iitafile kunye nemihlathi ziya kwenziwa ukususela apho ukufunda, isicelo kufuneka ucacise uhlobo umyalelo, uze ubone intsimi okanye amasimi ziya kukhethwa.

Umzekelo, ukuba Abantu table kufuneka ukufumana data evela emasimini Igama Age, kwangaxeshanye ukubonisa iziphumo ukuze alfabhethi umhlathi Igama, oko kuya kunceda ukwenza yombuzo ilandelayo: KHETHA Igama, Age FROM Abantu UMYALELO BY Igama.

Indlela ukubeka umyalelo hlobo?

iimeko zangoku yingozi badwelisi benkqubo neendidi zemisebenzi, yaye ngamanye amaxesha kuyimfuneko ukuba kusengaphambili indlela apho iziphumo ziya kuboniswa - ngokuthi unyuka okanye usihla umyalelo, alfabhethi okanye ukuze reverse? Kwaye leyo SQL hlobo ukuze ugqibe ngokudibanisa igama elingundoqo kwi umbuzo. Emva kokukhetha amasimi kunye neetafile apho ulwazi olufunayo ifunyenwe, kuyimfuneko ukongeza UMYALELO NGU, ngoko ucacise igama umhlathi apho ufuna ukulungisa.

Kuba oda reverse kufuneka ukhankanye igama emva DESC parameter. Ukuba ngaba izinto ezimbini lula kuyimfuneko ukuba ndlela, ngoko ke columns ezahlulwe iikoma, yaye olu luhlu kuqala xa endle unikwa kukuba eza kuqala kuluhlu. Kufuneka kuqatshelwe ukuba ulungelelwaniso eziqalelo in reverse order parameter DESC inikela intsimi omnye kuphela, emva koko igama lo igama, ngoko ukuba ufuna kuyimfuneko ukucacisa yonke imihlathi ekhethiweyo.

iindlela ezizezinye yokuhlela

Ukuba akukho ubukho bokusetyenziswa eyakhelwe-ngaphakathi SQL-lokhetho, unako ukubhala enye ubuchule ezaziwayo. Ukuba kuyimfuneko, ukuphumeza izinto fastest odola, kufuneka asebenzise indlela "uhlobo ngokukhawuleza", ngokusekelwe isiqingatha-waphula uluhlu. Kwakhona ilifa rankings ukuthandwa enkulu 'kwiqamza "apho izinto ezibekwe ngecala lize kwimeko ilungiselelo engachanekanga," HeapSort "uthumela isiqalelo likhulu ekupheleni kuluhlu, kwakunye" indawo yokufaka umfanekiso luni "ikutshoyo indawo isiqalelo ngasinye ke.

Ngesandla lokubhala le algorithm akayi kuvumela ukwandisa kakhulu isantya yokuhlela, Noko ke, kuya kuba negalelo kuphuhliso lwezakhono lwenkqubo, kodwa kuya kuvumela ukuguqula inkqubo ngokulungelanisa imeko-scheme ranking ukuba ledatha ethile ukuphucula ukusebenza kwale nkqubo.