Khetha isitatimende (SQL)

Dlulisa SQL isifinyezo ngokwayo (ulimi imibuzo ehlelekile) ibonisa iqiniso ukuthi idinga - evame kakhulu ukuba kusetshenziswe i-elementi e-SQL. Khetha imigqa, ukuqeda ngokuzenzakalelayo idatha eziphindaphindekayo, weqa noma oda kabusha amakholomu ukusiza Khetha (SQL) - isitatimende equkethe inkomba database ukuletha ulwazi oluthile.

-syntax

Ukusebenzisa kahle kwanoma yimuphi umqhubi kufanele uqale uzijwayeze syntax ulimi lokwakha enixoxa ngalo. Lapho ukhuluma ngokuqondile mayelana ulimi SQL, Khetha (opharetha) has syntax ezilandelayo:

Khetha	Likhuluma database ukuthi sidlula ngesicelo. Lokhu elisemqoka.
Nelinye ezintathu ...	Bhala ukubonisa amakholomu
Kusukela	Ikhombisa igama ithebula kuyiphi idatha ngeke ekhethiwe. Libuye elisemqoka saziso.

Lokhu okubizwa ngokuthi "short" opharetha miyalelo, kodwa lisibonisa yokuthi ngaphandle angukhiye Khetha bese kusuka database alihambisani isicelo sethu.

isitatimende-syntax Okugcwele iboniswa sibalo elandelayo:

Lapha Lapho isigatshana ikuvumela usesho lwakho lube ngcono ngokucacisa izimo.

Ukuze abakuzuzile ukubuthelela isicelo kubo uquqaba umsebenzi esetshenziswa sipho Group, futhi ukucacisa yi ngemva yokwenza iqembu isebenzisa Ukuba isiphakamiso.

Hlela uhlobo kuzokwenza amanani kwekholomu ekhethiwe in inyuka noma kwehle.

с информацией: Ukuze abone ngokucace kakhudlwana ukuthi nge Khetha isitatimende, ucabange ukuthi database yethu ine etafuleni Cats lwazi olulandelayo:

I-Id	Breed	Igama	usuku lokuzalwa oluhle	Umbala
1	Bobtail	nkosi	04/01/2017	Mpunga
2	Ukoma	feint	03/16/2017	White
3	Mau	panther	03/30/2017	Black
4	Bobtail	Tyson	02/23/2017	Mpunga
5	Burmilla	Athena	08/01/2017	Black

Umugqa ngamunye iqukethe igumbi kitty esiyingqayizivele, zohlobo yayo, isidlaliso, usuku lokuzalwa kanye Umbala. Sizobuye sidingide indlela opharetha Khetha (SQL), isivele esekelwe idatha kusuka kuleli thebula.

Njengoba idatha ezisuka kubantu etafuleni

Njengoba kushiwo ngenhla, ukuze uthole isampuli ulwazi oludingayo kusuka etafuleni sizosebenzisa angukhiye.

I elisemqoka icacisa Khetha amakholomu ukubonisa. Ungenza uhlu amakholomu edingekayo, kuhlukaniswe ukhefane, khona-ke wonke isakhiwo izobukeka kanje:

Khetha umbala, mhlawumbe ngohlobo, igama Kusukela Cats

Njengoba ubona, thina nihlela amakholomu ngokulandelana bayasidinga. Ngaphezu kwalokho, singaba ngokuthini kuphela amakholomu kudingekile kithi.

Kukhona ifasitela lokuzula kuwo wonke amakholomu ekubukweni kwethebula. Ukuze wenze lokhu, emva Khetha abonisa asterisk (*) ngesikhala. Sonke lesi sakhiwo izobukeka kanje:

Khetha * Kusukela Cats

Waba yini umphumela wesibindi nombuzo ngenhla - wonke etafuleni amakati, ethulwa ngesimo lapho kuqukethwe ekupheleni kwesigaba sokugcina.

Abantu abaningi bayazibuza ukuthi esikhundleni ukusebenza SQL Khetha ilandelana. Ngokuvamile lokhu iyadingeka uma ufuna ukuhlanganisa isibongo, igama patronymic lomuntu afakwe amakholomu ahlukaniswe.

Kulokhu, sihlanganisa izinhlobo kanye nemibala zamakati Cats etafuleni. Imininingwane ehilelekile ukuthi yolwazi ehlukene osetshenziselwa concatenation umucu wezinhlamvu ezahlukene. Kwezinye izimo kungcono nje plus (+), kwesinye - a double umugqa sici (||) noma i-ampersand (&), ngezinye izikhathi asetshenziswa futhi we- CONCAT. Ngakho-ke, ngaphambi ngokuhlanganisa isidingo ukufunda isichasiselo database ethize ngawo ukusebenza.

Khetha uhlobo || ',' || umbala Kusukela amakati

Umphumela sithola lokhu okulandelayo:

Breed, Umbala

Bobtail, Mpunga

Ukoma, White

Mau, Black

Bobtail, Mpunga

Burmilla, Black

Ukuqedwa idatha eziphindaphindekayo

Izindlala - amafuction (SQL), okuvumela ukuba sigweme ukuphindaphindeka kochungechunge ngokuphelele ezifanayo imiphumela isampula.

Ngokwesibonelo, sifuna ukwazi ukuthi izinhlobo cat ube etafuleni lethu. Uma usebenzisa nombuzo elula:

Khetha uhlobo Kusukela Cats

Khona-ke sithola yi kulindeleke:

Breed

Bobtail

Ukoma

Mau

Bobtail

Burmilla

Njengoba ubona, Bobtail zohlobo liphindwe kabili kabili. agumenti Izindlala ukugwema ukuphindaphindeka, sidinga kuphela ukwengeza umbuzo:

Khetha uhlobo abahlukene Kusukela Cats

isicelo Ukucacisa

Empeleni, cishe isicelo noyedwa kungaqali okukhipha idatha njengoba isethi ephelele imigqa kuthebula. Cabanga isiphakamiso Khetha (SQL) kuzovumela ukuchaza inqubo ukukhethwa nje emigqeni efanele.

Ngakho-ke lokhu impahla kuphi. Isiphakamiso osebenza isilandiso - inkulumo okunemibandela onikeza ukubaluleka wokukhipha "iqiniso" noma "amanga." Khetha isitatimende ngeke kukhishwe kuphela idatha kusuka etafuleni okuyiwona isisho esinombandela kuzodingeka ku-Iqiniso, noma "iqiniso".

Kuyasiza idili nale ukwakhiwa, isampula elula. Ake sithi sifuna ukwazi konke mayelana amakati umbala omnyama.

Khetha * Kusukela amakati Lapho color = 'Black'

Waba yini umphumela walesi nombuzo kuyoba umugqa olandelayo we ithebula:

3	Mau	panther	03/30/2017	Black
5	Burmilla	Athena	08/01/2017	Black

Ungase futhi sihlanganise izimo ngokusebenzisa opharetha okunengqondo Futhi, Noma, Hhayi.

Ukunikeza Qoqa nge-

Ukunikeza Qoqa nge-, elisetshenziswa Khetha (SQL), ikuvumela ukuba iqembu le sombuzo ukubaluleka ikholomu ethize (noma amakholomu), bese sizisebenzise kumsebenzi ekuhlanganisweni.

Ukuze imisebenzi aggregate zihlanganisa:

• Bala - sinquma inani lemigqa akhethwe nombuzo.
• Sum - isamba izibalo zonke kukholamu ekhethiwe.
• Min - okukhiphayo ubuncane kukholamu ekhethiwe.
• Max - ngokulandelana, amagugu esiphezulu kukholamu ekhethiwe.
• Esimaphakathi - inani eliyisilinganiso.

Isikimu isiphakamiso kuqondakala kangcono kakhulu ngendlela isibonelo. Ake sithi sifuna ukuthola ukuthi ingakanani ngayinye amazinyane esinalo zohlobo. Ukuze wenze lokhu, dala nombuzo elandelayo elula:

Khetha uhlobo, ukubala (*) Kusukela amakati Qoqa nge- zohlobo

Waba yini umphumela wesibindi intando ethebuleni elandelayo:

Breed	Bala
Bobtail	2
Ukoma	1
Mau	1
Burmilla	1

Njengoba ubona, bobtail amakati ukuzalela esinawo ezimbili, abanye njengoba eyodwa nje. Ngesicelo umkhuba onjalo, okusekelwe etafuleni lethu, umfuyi bakwazi ukuzwisisa lokho amakati izinhlobo bayindlala phakathi amakhasimende, futhi yini - akukho.

Kungenzeka ukuthi, ngenxa yesibalo esikhulu okufakwe etafuleni langempela usafuna ukucacisa isicelo futhi sibonise kuphela labo amazinyane, okuyinto ayengajuli ngaphezu, isibonelo, eziyishumi. Ukuze sicacise noma amaqembu isihlungi isetshenziswa Ukuba isiphakamiso. Kusenza sikwazi ukuzwa ukuthi ukulahla amaqembu athile, afana isigatshana kuphi, okuyinto ewalahla imigqa ethile. Isimo ebizwa ngokuthi umsebenzi sesisonke. Gcwalisa isicelo:

Khetha uhlobo, ukubala (*) Kusukela amakati Qoqa nge- zohlobo Ukuba ukubala (*) <= 10

Njengoba isimo sacelwa "isibalo amakati of zohlobo ngalunye akuyona engaphezu kuka-10", umphumela sithola kuyafana kulesibonelo, ngaphandle ecacisa. Kodwa lapha kubalulekile ukuqonda uhlelo uqobo isebenza Ukuba isiphakamiso. Kodwa uma ushintsha isimo logic ku Ukuba ukubala (*) = 1, bese umphumela kuzoncishiswa obuthathu bese ubonisa amazinyane, ezashiya eyodwa nje.

ukuhlunga

Jwayelana Order by - sipho Khetha (SQL) opharetha evumela ukuhlunga okukhipha imigqa in inyuka noma kwehle ngo eyodwa noma ngaphezulu izinsika.

Kubalulekile ukukhumbula ukuthi i-oda ngo- - kuba umnikelo wokugcina zonke Khetha isitatimende isakhiwo. It ibekwe emva Khetha, Kusukela, kuphi, Qoqa nge-, Ukuba.

Kukhona amaphuzu amathathu aqakathekileko ngesikhathi Ukuhlelwa:

1) Ungaveza iyiphi inombolo yamakholomu, ngalinye elikwazi ngamunye angase ahlungwe noma sinyuka (ASC), noma ukwehla (DESC).

2) Zonke amakholomu ngenhla Order umusho kufanele zibe khona phakathi amakholomu akhethiwe ku Khetha.

3) uhlu Akuphoqelekile amagama athile ukuhlunga amakholomu, ungavele ukhombe izinombolo zabo ngaphansi lapho ukuya Khetha isitatimende.

Sithemba ukuthi ngokusebenzisa lesi sihloko ngeke uthole ulwazi oluyisisekelo mayelana nokuthi isetshenziswa kanjani imibuzo SQL futhi manje ukhethe imininingwane edingekayo kusukela database wakho kalula.