SQL Kuphi: izindlela kwesicelo nezibonelo

ngokusebenzisana SQL yolwazi kubhekwa omunye ezivamile izilimi izinhlelo. umklamo limi alukwazi kuphela Ukwakha database, kodwa futhi ukuchitha isikhathi manipulations yakhe ehlukahlukene on the ushintsho idatha noma isampula.

Ukukhetha idatha kusuka database usebenzisa Khetha design [idatha isethi] kusuka ku- [igama lethebula]. Okuhlangenwe nakho kuye kwabonisa ukuthi u-80% amacala ukusetshenziswa khetha imibuzo, idatha okufanele isetshenziswe ezihlukahlukene izimo - izilungiselelo. Ukuze wenze lokhu ngolimi ngaphezu isicelo, ukuba yinkimbinkimbi kwako isimo ayisungula SQL-Lapho.

Izindlela Yokusebenzisa Lapho izimo

Kaningi, uMklami Kufanele kukhethwe ngokuyinhloko ukunikeza umbiko, idatha egcinwe kwi-database. Lokhu kungase kunganeli ukwakha khetha umbuzo olula. Njengomthetho, kumelwe acabangele ezihlukahlukene izimo, izibonelo zalokho imingcele, okungaba impela eningi, noma hlola. uma idatha zivezwe ububanzi noma itafula ethile.

SQL-Lapho design ingasetshenziswa ukusetha idatha izibonelo zalokho noma izimo ukuqinisekisa nesenzakalo idatha amasampuli noma izinhlangothi etafuleni.

Lapho ukusetshenziswa yokuqondisa nemingcele amasampuli

Uma ufuna ucacise nemingcele ethile ukuze ukhethe database yokubika, SQL-Lapho design-syntax zingahlelwa ngempela kalula. Ukuze wenze lokhu, ungasebenzisa le mithetho elandelayo:

1. Sidinga prostroen isicelo olujwayelekile Khetha * kusuka ukwakhiwa.

2. Ezimisele usebenzisa ukhiye Joyina izakhiwo ngalo amatafula enze ukukhetha.

3. Nge isakhiwo kuphi usethe uhlu ipharamitha isampula.

Lolu hlobo isicelo ulula ekwakheni futhi akabubangeli nobunzima ngisho Wabasaqalayo.

Ukusebenzisa isakhiwo ukuqinisekisa okuvela kuyo

Uma uhlelo wajuba hhayi kuphela khetha kusuka idatha ngesimo etafuleni, kodwa futhi ukuze uhlole okufakile yawo esicini esisodwa noma ngaphezulu amatafula kwenye plan, SQL-Lapho design lubaluleke.

Nale ukwakhiwa, syntax bangakha kwezimo ezifanayo, kodwa imibuzo isichazi, ezosheka okuvela kuyo imigqa ekhethiwe e iqoqo third-party semininingwane amatafula.

Njengomthetho, le njongo kwakhiwa itafula lesikhashana ukuqopha yonke kudingekile, ukuqinisekisa okuvela kuyo isethi yedatha.

Izibonelo yokusetshenziswa kuphi

Ubani ukunikeza izibonelo kuphi SQL. Ngaqala ukucabanga ukuthi kukhona amatafula amabili ngedatha - Tovar futhi TovarAmount. Esikhathini sokuqala, kukhona amagama izimpahla ithengiswe, intengo, usuku ukudayiswa kanye ikhasimende abathenge umkhiqizo. Esikhathini yesibili kuboniswe khona izimpahla, noma kunalokho lokho luyatholakala.

Isibonelo ipharamitha nombuzo, ecacisa zonke izinto ethengisiwe ethile izinsuku kuyoba lezi zinhlobo ezilandelayo design:

Khetha * kusuka Tovar

Lapho T_Date> = '12 / 01/2016 'futhi T_Date <=' '12 / 07/1016 kanye T_PriceOut> 0

uhlelo ezinjalo nombuzo ibuyisa uhlu imikhiqizo, amathebula edatha ababethengiselwe ezinsukwini zokuqala eziyisikhombisa Disemba, njengoba kuboniswa omunye izimo izibonelo zalokho: T_PriceOut> 0.

Uma sicabangela isimo okukhipha kwezimpahla ezisebusweni isitokwe, ukwakhiwa plan onjalo uyoba:

Khetha * kusuka Tovar

Lapho T_Tovar e (khetha TA_Tovar lapho TA_ Inani> 0)

Le mpophoma izimo Lapho kungenzeka kunabaningi, kodwa kuyafaneleka ukusho ukuthi izimo abaningi ozibekile, isikhathi eside uzogcwala nombuzo. Lokhu kwaba isizathu ukusetshenziswa amathebula zesikhashana. Okuningi ngokushesha ukukhiqiza eziningana zalokhu, bese uqhathanisa idatha kuzo, kunalokho ukwakha isimo nge amazinga multiple of ukuqinisekiswa idatha.