SQL eziphawulekayo: inkcazelo, imizekelo, iimpawu

Amaxesha amaninzi xa usebenzisa SQL kuyo ze kuvunjululwe idata esuka iitafile, umsebenzisi ifumana idata ezingasasebenziyo ke khona i phindo imiqolo ezifanayo ngokupheleleyo. Ukuze siphephe le meko, sebenzisa SQL ingxabano eyahlukileyo kwisivakalisi Khetha. Eli nqaku liza kuxubusha imizekelo ngokusetyenziswa kwale ngxoxo, kwakunye iimeko apho isicelo kufuneka bawulahle ingxabano.

Ngaphambi kokuba uqhubeke ukuba siqwalasele imizekelo ezithile, ukwenza i sedata efunekayo isibini nezingqengqelo.

itheyibhile kulungiselelwa

Masithi sibe lugcina ulwazi database malunga lodonga achazwe iitafile ezimbini. Le oboi itheyibhile (lodonga) kunye namasimi id (isazisi esisodwa), lohlobo (uhlobo lodonga -. Iphepha, IVinyl, njalo-njalo), umbala (umbala), ukuba umthetho (isakhiwo) kunye nexabiso (ixabiso). Ke itafile Ostatki (iintsalela) kunye namasimi id_oboi (a kubhekiswa isazisi esisodwa kwitheyibhile Oboi) kunye count (inani adlula kwi stock).

Gcwalisa le theyibhile data. Kwitheyibhile ukongeza lodonga kwamarekhodi 9:

Itheyibhile kunye iintsalela - neerekhodi anesithoba:

Sichubeka njani nenkcazelo komyalelo awathethi SQL.

Beka ezahlukeneyo Khetha kwigatya

ingxabano hluke kufuneka zibekwe nangoko emva Khetha igama elingundoqo kwi imibuzo. Wayewabhekisa yonke imihlathi ezicaciswe kwisivakalisi Khetha, kuba uya ngaphandle isiphumo umbuzo lokugqibela imitya twatse ngokupheleleyo. Ngenxa yoko, kanye kwanele ukuba ucacise xa ubhala SQL «khetha eyahlukileyo» isicelo. Le Okungumahluko ukusetyenziswa kwemisebenzi aggregate ngaphakathi enomahluko ukuba ujonga kancinane emva koko.

Kufuneka kukhunjulwe ukuba uninzi ledatha yaye ayisazi uhlobo yakho yesicelo:

Akukho ibingento impikiswano izihlandlo eziliqela okanye kanye elikhankanyiweyo, kodwa phambi kokuba lwesibini, lwesithathu okanye omnye umhlathi ekhethiweyo. Uza kufumana i impazamo wayebhekisela impazamo yokuma kwezivakalsi.

imibuzo ecacileyo isicelo eqhelekileyo

Kucacile ukuba olufanelekileyo ukwakha iitafile isakhiwo ukuzigcwalisa ngaphakathi itafile enye ngaphandle xiyimo xa kukho imitya twatse ngokupheleleyo. Ngoko ke, ukwenziwa umbuzo «Khetha enomahluko *» kunye isampuli etafileni enye akunakwenzeka.

Cinga ngemeko xa kufuneka sazi ukuba uhlobo sinayo iphepha lodongo, nje lula, ukuba kuhlelahlelwe hlobo:

Kwaye bafumane iziphumo:

Njengoko sibonile kule theyibhile kukho nempinda imiqolo. Xa sifakela isindululo Khetha eziphawulekayo:

sifumana umphumo ngaphandle ukuphindaphinda:

Ngoko ke, ukuba ukungena kakuhle data kwi itafile, ngoko nangoko emva umnxeba okanye ngesicelo abathengi singakwazi ukuphendula loo nto ilulwelo iphepha lodongo, fiberglass kunye acrylic lodongo ziyafumaneka kwi evenkileni abekho. Sisazi ukuba uluhlu ezivenkileni kudla ayiphelelanga nekhulu iphepha lodongo, ukujonga uluhlu lweentlobo non-ubalaseleyo wayeza kuba kakhulu ekusekweni kwemisebenzi.

Isicelo lwemisebenzi aggregate enomahluko ngaphakathi

ingxabano SQL hluke zingasetyenziswa nawuphi na umsebenzi othunyelweyo. Kodwa i Min kunye Max isicelo sayo iya kuba nafuthe, kodwa xa kubalwa inani okanye ixabiso avareji Akuvamile ukuba kwimeko apho kungekho bani kufuneka ithathele ingqalelo kophindo.

Masithi ufuna ukukhangela umthamo yokugcina lwethu kwaye thumela olu isicelo, computes inani elipheleleyo ohlobo stock:

Cela uya kunikela impendulo 143. Ukuba, ke, siya kutshintsha ku:

ukuba bafumane lilonke 119, njengoko lodonga amanani inxenye 3 no-7 sele stock kwi mali efanayo. Noko ke, kuyabonakala ukuba impendulo ngu engalunganga.

Eyona isoloko isetyenziswa kwi SQL umsebenzi ecacileyo Count. Ngoko ke, kulula ukufumana iintlobo ezininzi kangakanani elikhethekileyo iphepha lodongo, siba:

Kwaye ufumane isiphumo 5 - iphepha eziqhelekileyo kabini-maleko IVinyl kunye nelaphu non-ephothiweyo. Ngokuqinisekileyo wabona lo mhlomlo ezifana: "Kuphela ukuba iintlobo ezingaphezu kwe-20 ka iphepha lodongo," yi leyo ethetha ukuba le venkile yi rolls nje isibini nedazini iintlobo lodonga ntlobo zale mihla.

Kunika umdla ukuba lo mbuzo mnye, ungakhankanya imisebenzi ezininzi ezifana Count bathi ezicace, kwaye ngaphandle kwayo. Yiloo nto ke imeko kuphela apho ezahlukeneyo Select'e kunokuba amaxesha langoku eziliqela.

Xa ukulahla ukusetyenziswa Kwempikiswano

Ukususela ukusetyenziswa SQL argument ecacileyo kufuneka kushiywa kwenye iimeko ezimbini:

1. Wena ukwenza sokukhetha iitafile kwaye eqinisekile ixabiso esisodwa ngasinye. Kulo mzekelo, ukusetyenziswa argument alilungile, ngenxa yokuba umthwalo owongezelelweyo kumncedisi okanye iklayenti (ngokuxhomekeke kuhlobo DBMS).
2. Ngaba ukoyika ukulahlekelwa data yakho. Makhe sibone ukuchaza.

boss Masithi likucela ukuba uluhlu lodonga ukuba unayo, kunye isalathiso ntsika zimbini kuphela - uhlobo kunye nombala. Out of mkhwa, unika ingxabano eziphawulekayo:

Kwaye - ulahlekelwe ezinye iinkcukacha:

Mhlawumbi bacinge ukuba lodonga iphepha (eliqhelekileyo kwaye ezimbini-maleko) siye banasicamango sinye kuphela, enyanisweni, nkqu itafile elincinane nqaku amabini (kukhokelela ngaphandle enomahluko):

Ngoko ke, njengokuba ngembalelwano nasiphi na isicelo kunye ingxabano imfuneko ecacileyo ukuba alumke yaye babenolwazi nobuchule xa isigqibo ngesicelo yayo, kuxhomekeka msebenzi.

enye hluke

Ngokwahlukileyo ingxoxo enomahluko - All ingxabano. Xa isicelo yayo phindo kuqalwa zigcinwa. Kodwa njengoko iziko ledatha engagqibekanga waza wafumanisa ukuba oko kuyimfuneko ukubonisa zonke amaxabiso, lo argument All - oko kunokuba a Nations ngaphezu umsebenzi yingxoxo elililo. Siyathemba ukuba ngoku uqonde ukuba ahlukile (SQL) iyasetyenziswa. Inkcazelo akunike inkcazelo epheleleyo malunga nokwenzeka usebenzisa le ngxoxo ekusombululeni iingxaki ezahlukeneyo. Kakade ke, njengoko kwaba, nkqu ingxabano elula efana kwisicelo sayo ifihla ithuba obubonakalayo kakhulu yokuphulukana ezinye iinkcukacha kunye nokubonisa ulwazi olungachanekanga.