Video: Vim li cas peb thiaj siv CTE hauv SQL Server?
2024 Tus sau: Lynn Donovan | [email protected]. Kawg hloov kho: 2023-12-15 23:47
Dab tsi yog a CTE los yog Common Table Expression hauv SQL Server ? A CTE ( Common Table Expression ) txhais cov txiaj ntsig ib ntus teeb uas koj ua tau ces siv hauv ib nqe lus SELECT. Nws dhau los ua ib txoj hauv kev yooj yim los tswj cov lus nug nyuaj. Common Table Expressions yog txhais nyob rau hauv nqe lus siv nrog tus neeg teb xov tooj.
Ib yam li ntawd, tib neeg nug, thaum twg kuv yuav tsum siv CTE hauv SQL Server?
Vim li cas rau siv a CTE Hauv SQL ntawv , peb yuav siv sub-queries los koom nrog cov ntaub ntawv los yog lim cov ntaub ntawv los ntawm sub- nug . Thaum twg peb xa cov ntaub ntawv qub lossis koom nrog tib cov ntaub ntawv teev tseg siv ib sub- nug , kev ruaj ntseg code yuav nyuaj. A CTE ua kom zoo nyeem ntawv thiab kev saib xyuas yooj yim dua.
Tom qab ntawd, lo lus nug yog, koj ua CTE li cas? Koj tuaj yeem siv a CTE hauv a Tsim ib qho kev pom, raws li ib feem ntawm qhov saib qhov SELECT query. Ntxiv rau, raws li SQL Server 2008, koj tuaj yeem ntxiv ib qho CTE mus rau MERGE cov lus qhia tshiab. Tom qab koj txhais koj cov lus nrog CTEs, koj tuaj yeem xa mus rau CTEs raws li koj xav xa mus rau lwm lub rooj.
Yog li ntawd, qhov zoo ntawm kev siv CTE hauv SQL Server yog dab tsi?
CTE siv los hloov qhov saib uas khaws cov metadata. CTEs pab txhim kho kev nyeem ntawv ntawm cov cai yam tsis muaj kev cuam tshuam kev ua haujlwm. Lawv pab txhim kho kev ruaj ntseg ntawm cov cai yam tsis muaj kev cuam tshuam kev ua haujlwm. Lawv sau cov code recursive hauv T- SQL ntawv yooj yim dua li yav dhau los SQL Server cov qauv.
Puas yog CTE txhim kho kev ua haujlwm?
Ib qho tseem ceeb sib txawv yog qhov optimizer ua tau siv cov txheeb cais los ntawm lub rooj ib ntus los tsim nws cov lus nug. Qhov no ua tau tshwm sim hauv kev ua tau zoo tau txais. Tsis tas li, yog tias koj muaj qhov nyuaj CTE (subquery) uas yog siv ntau tshaj ib zaug, ces khaws cia rau hauv lub rooj ib ntus yuav feem ntau muab ib txhawb kev ua haujlwm.
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