Isitatimende se-Select (SQL)

Ukuguqulwa kwesicatshulwa seSQL (ulwimi lwemibuzo ehleliweyo) kubonisa ukuba imibuzo iyona nto isetyenziswa rhoqo kwiSQL. Khetha imida efunekayo, ngokuzenzekelayo ungafaki idatha evulekileyo, yeqa okanye ulungise kwakhona iikholomu ziza kunceda Khetha (SQL) - umsebenzisi oxelela i-DBMS ukukhupha ulwazi oluthile.

Syntax yoMqhubi

Ukusebenzisa ngokufanelekileyo nayiphi na umsebenzisi, kufuneka uqale uzifundele nge-syntax yolwimi olukhankanyiweyo. Xa sithetha ngokukodwa malunga nolwimi lwe-SQL, khetha (umqhubi) une syntax elandelayo:

Khetha	Uxelela isiseko sokuba sisithumela isicelo. Eli lizwi eliphambili.
Omnye, amabini, abathathu ...	Uluhlu lwamakholomu wokuvelisa
Ukusuka	Ibonisa igama letafile apho ukhetha khona idatha. Kwakhona negama elingundoqo.

Le yinto ebizwa ngokuthi "mfutshane" ye-syntax yomsebenzisi, kodwa ibonisa ukuba ngaphandle kokukhetha amagama angundoqo kunye ne-DBMS, umbuzo wethu awuyi kuwenza.

I-syntax epheleleyo yomqhubi iboniswa kulo mfanekiso olandelayo:

Apha, apho igatya likuvumela ukuba uhlaziye uphando ngokucacisa imeko.

Ukuqulunqa amaxabiso kunye nokusebenzisa umsebenzi ohlangeneyo kubo, iQela ligqitywa ngumhlathi, kwaye icacise umphumo emva kokuqoqa, isiphakamiso Emva kokusetyenziswa.

Umyalelo ngokuya kukuvumela ukuba uhlalutye amanani eikholomu ezikhethiweyo ukwenyuka okanye wehla.

с информацией: Ukufumana ukuqonda okungcono kwe-Option operator, makhe sicinge ukuba kwisiseko sethu setafile kukho itafile ezilandelayo zeekati ngolwazi:

Id	Uzala	Igama	Lokuzalwa	Umbala
1	Bobtail	Nkosi	04/01/2017	Grey
2	Curl	Fint	03/16/2012	Mhlophe
3	Mau	Panther	03/03/2017	Omnyama
4	Bobtail	Tyson	02/23/2017	Grey
5	Burmala	Athena	01/08/2017	Omnyama

Umqolo ngamnye wetafile uqulethe inombolo ekhethiweyo yenkiti, ubuhlanga, isiteketiso, umhla wokuzalwa kunye nombala. Oku kulandelayo, siza kuqwalasela indlela umqhubi we-Khetha (SQL) osebenza ngayo, ngokusekelwe kwidata esuka kule tafile.

Indlela idatha ngayo kwitafile

Njengoko kuchaziwe ngasentla, amagama angundoqo asetyenziswa rhoqo ukukhetha ulwazi oluchanekileyo etafileni.

Emva kokukhethwa kwegama elingundoqo, iikholomu zemveliso zichazwe. Unokwenza uluhlu lwamakholomu afunekayo ahlukaniswe yi-comma, kwaye isakhiwo sonke siya kubonakala ngathi:

Khetha umbala, udidi, igama Iikati

Njengoko ubona, sinokuhlela amakholomu ngendlela efuneka ngayo. Ukongeza, sinokubonisa kuphela iikholomu esizifunayo.

Kukho ukungena okufutshane ukujonga zonke iikholomu zetafile. Ukwenza oku, emva kokukhetha, i-asterisk (*) iboniswe ngesithuba. Ukuqulunqwa konke kuya kubonakala ngathi:

Khetha * Iikati

Isiphumo salo mbuzo ukhankanywe ngasentla yiyonke itheyibhile yeekati, imelwe njengoko ibonakala ekupheleni kwecandelo lokugqibela.

Abantu abaninzi banomdla wendlela yokubeka iziphumo zokuphunyezwa kwi-SQL Khetha kwintambo. Ngokuqhelekileyo oku kufunwa xa kuyimfuneko ukudibanisa inqampu, igama lokuqala kunye nomntu ongu-patronymic womntu, efakwe kwiikholomu ezingenayo.

Kwimeko yethu, sidibanisa udidi kunye nombala weekati ezivela kwiitheyibhile. Ubungqina bokuba i-DBMS eyahlukileyo isebenzisa abalinganiswa abahlukeneyo ngomtya wokuncintisana. Kwezinye iimeko, oku kuhlanganiswa (+), kwezinye - umgca ombini (||) okanye i-ampersand (&), ngamanye amaxesha i-Concat operand isetyenziswa. Ngoko ke, ngaphambi kokudibana, kufuneka ufunde isichaso kwiDBMS ethile osebenza kuyo.

Khetha udidi || ',' || Umbala Iikati

Isiphumo silandelayo:

Uhlobo, Umbala

Bobtail, Grey

Curl, White

Mau, Black

Bobtail, Grey

Burmilla, Black

Ingafani nedatha ehlukileyo

Umsebenzi ohlukileyo - Khetha (SQL), evumela ukuba ungabandakanyi ukuphindaphindwa kwemigca efana neyona ndlela evela kwisiphumo.

Umzekelo, sifuna ukufumana ukuba yeyiphi ikati etafileni lethu. Ukuba sisebenzisa umbuzo olula:

Khetha udidi Iikati

Ukuba siya kufumana umphumo olindelekileyo:

Uzala

Bobtail

Curl

Mau

Bobtail

Burmala

Njengoko ubona, uhlanga lweBobtail luphindwe kabini. Ukwahlukana kweNkqubela kuya kuphelisa ukuphindaphinda, kufuneka nje ugcwalise umbuzo:

Khetha uhlobo oluhlukileyo Iikati

Ukuhlaziywa kwesicelo

Enyanisweni, phantse akukho mbuzo wokuphuma kwedatha njengesiqhelo esipheleleyo semigca yeetafile. Makhe siqwalasele, sisiphi isigwebo kwi-Select (SQL) sivumela ukusetha imigaqo yokukhetha kuphela imigca efunekayo.

Kulapho. Kulo mvakalisi, isalathiso isetyenzisiweyo-inqobo engqinelanayo eyenza ukuveliswa kwexabiso "lokwenyaniso" okanye "lobuxoki". Khetha inkcazo ifumana kuphela ezo nkcukacha ezivela etafileni apho ibinzana elingqinelanayo liza kuNyaniseka, okanye "lokwenyaniso."

Kuya kunceda ukuhlenga ngelo lwakhiwo isampuli elula. Masithi sifuna ukwazi konke malunga namakati amnyama.

Khetha * Iikati Umbala = 'Umnyama'

Umphumo wale mbuzo uza kuba yimigca elandelayo yetafile:

3	Mau	Panther	03/03/2017	Omnyama
5	Burmala	Athena	01/08/2017	Omnyama

Unokudibanisa kunye neemeko usebenzisa kwaye, Okanye, Abenzi abasebenzisi bemilo.

Iqela

Iqela eligqityiweyo, elisetyenziswe kwi-Select (SQL), likuvumela ukuba uqoke imibuzo ngokubaluleka kwikholam ethile (okanye iikholomu), uze usebenzise umsebenzi opheleleyo kubo.

Imisebenzi ehlanganisiweyo ibandakanya:

• Bala - ibala inani lemiqolo ekhethwe ngumbuzo.
• Sum is summary sum sum of all values.
• Ukuncinciza ubuncinci ubuncinane beempawu zekholomu ezikhethiweyo.
• Ubuninzi bobukhulu beempawu zekholomu ezikhethiweyo, ngokulandelanayo.
• I-Avg ingumyinge.

Isicwangciso solu siphakamiso siqondwa ngokulula kumzekelo okhonkco. Masithi sithi sifuna ukwazi ukuba zingaphi iikiti zohlobo ngalunye esinalo. Ukwenza oku, kufuneka uvelise le mibuzo elula:

Khetha uhlanga, ubale (*) Iikati Iqela ngohlobo

Isiphumo sokwenziwa komsebenzi siya kuba yileta elandelayo:

Uzala	Bala
Bobtail	2
Curl	1
Mau	1
Burmala	1

Njengoko uyakubona, sinemibhobho emibili ngeekiti, abanye banalo kuphela. Ngokwenza kwesi sicelo, esekelwe kwitheyibhile yethu, umfuyi unokuqonda ukuba yeyiphi intlobo zeekati ezifunekayo kubathengi, kwaye ezingekho.

Kungenzeka ukuba ngenxa yenani elikhulu lokungena kwitafile yangempela, kuya kusadingeka ukuba kucacise umbuzo kwaye uhoxise kuphela ezo ntlobo zeekiti, ezingabikho, umzekelo, ezilishumi. Ukucoca okanye ukucoca amaqela, sebenzisa ukuba ube neCandelo. Ikuvumela ukuba ulahle amaqela athile, afana nalapho Apho isigatya, esilahla imigqa nganye. Umqathango unikezelwa ngumsebenzi wonke. Siza kwongeza umbuzo:

Khetha uhlanga, ubale (*) Iikati Iqela ngohlobo Ukubala (*) <= 10

Ekubeni imimiselo esiyibekayo "inani leenkiti zohlobo ngalunye alingekho ngaphezu kwe-10", umphumo ufana nomzekelo ngaphandle kweenkcukacha. Kodwa ke kubalulekile ukuqonda oko kanye isicwangciso somsebenzi wesiphakamiso. Kodwa ukuba sitshintsha imeko enengqiqo Ukuba nebala (*) = 1, ngoko umphumo uya kuncitshiswa ibe yimigca emithathu kwaye ivelise ukuzaliswa kwamakhiteni, enye enye eyashiywe.

Ukuhlela

Masiqhelane noMyalelo nge-isivakalisi somqhubi we-Select (SQL), ovumela ukuba uhlele imigca yokuvelisa ngokunyuka okanye ukunciphisa imilinganiselo kwikholam enye okanye ngaphezulu.

Kubalulekile ukukhumbuza ukuba iMyalelo yiyo isivakalisi sokugqibela sayo yonke indawo yokwakhiwa kweSitethi. Kubekwe emva koKhetho, Ukusuka, Kuphi, Iqela,, Ukuba.

Xa uhlela, kukho iingongoma ezintathu ezibalulekileyo:

1) Ungacacisa nayiphi na inamba yamakholomu, elowo lingahlulwa ngokunyukayo okanye lenyuka (ASC) okanye luhla (DESC).

2) Zonke iikholomu ezikhankanywe kwiSigqibo ngumhlathi kufuneka zibe khona phakathi kweikholamu ezikhethiweyo kwi-Select.

3) Akukho imfuneko ukuba uluhlu lwamagama ekholomu abathile ekuhlunguleni, unokwazi ukucacisa amanani abo, ngaphantsi kokuya kuwo Khetha iSitatimende.

Sithemba ukuba ngoncedo lolu nqaku ufumene ulwazi olusisiseko malunga nokusebenzisa imibuzo ye-SQL kwaye ngoku ungakhetha kalula ulwazi oluyimfuneko kwi-DBMS yakho.