Yog, raws li kev siv DISTINCT yuav (qee zaum raws li cov lus pom) ua rau tau txais kev txiav tximKev txheeb ntau pua cov ntaub ntawv yuav siv sijhawm. Sim GROUP LOS NTAWM tag nrho koj cov kab ntawv, qee zaum nws tuaj yeem ua rau cov lus nug optimiser xaiv qhov kev ua haujlwm zoo dua (tsawg kawg nrog Oracle Kuv pom qhov kev ua tau zoo tseem ceeb).
Puas yog siv qhov sib txawv?
Zoo, kev siv tsis raug ntawm "kev sib txawv" tsis yog tsuas yog zais qhov teeb meem tiag tiag (cov khoom sib tw hauv cov rooj, tsis muaj qhov xwm txheej hauv cov lus qhia) raws li tau hais los saum toj no tab sis kuj degrades query performance… Qhov no yuav ua rau tus nqi IO (kev nyeem cov lus nug) ntawm cov lus nug nce mus ntxiv.
Puas sib txawv ua rau cov lus nug qeeb?
Cov lus nug tsawg heev tuaj yeem ua tau sai dua hauv SELECT DISTINCT hom, thiab tsawg heev yuav ua qeeb dua(tab sis tsis qeeb qeeb) hauv SELECT DISTINCT hom tab sis rau cov ntaub ntawv tom qab nws zoo li tias daim ntawv thov yuav tsum tau tshuaj xyuas qhov sib npaug, uas hloov qhov kev ua tau zoo thiab lub nra hnyav rau daim ntawv thov.
Nws puas zoo dua los siv qhov sib txawv lossis GROUP BY?
Nyob hauv MySQL, DISTINCT zoo li nrawm dua GROUP BY yog tias qhov Field tsis tau ntsuas. DISTINCT tsuas yog tshem tawm cov kab sib npaug tab sis GROUP BY zoo li txheeb lawv ntxiv.
Vim li cas peb yuav tsum tsis txhob siv qhov txawv hauv SQL?
Yog xaiv qhov txawv yog nyob rau 'kho' ib qho teeb meem ces koj yuav tau txais kev ua haujlwm tsis zoo rov qab los. GROUP BY tso cai rau koj siv cov haujlwm sib sau ua ke, xws li AVG, MAX, MIN, SUM, thiab COUNT. DISTINCT tsuas yog tshem tawm cov duplicates.
