Video: Puas muaj cov ntaub ntawv SQL?
2024 Tus sau: Lynn Donovan | [email protected]. Kawg hloov kho: 2023-12-15 23:47
Cov SQL EXISTS Tus neeg ua haujlwm
Cov EXISTS tus neeg teb xov tooj yog siv los kuaj rau lub muaj nyob ntawm tej sau tseg hauv subquery. Cov EXISTS tus neeg teb xov tooj rov qab muaj tseeb yog tias cov lus nug xa rov qab ib lossis ntau dua cov ntaub ntawv.
Hais txog qhov no, koj yuav ua li cas xyuas seb puas muaj cov ntaub ntawv muaj nyob hauv SQL?
Rau kuaj seb puas muaj kab hauv a MySQL rooj los yog tsis, siv muaj nyob mob. Cov muaj nyob mob tuaj yeem siv nrog subquery. Nws rov qab muaj tseeb thaum muaj kab nyob rau hauv lub rooj, txwv tsis pub dag yog rov qab los. Muaj tseeb yog sawv cev nyob rau hauv daim ntawv ntawm 1 thiab cuav yog sawv cev li 0.
Ib sab saum toj no, puas muaj nyob hauv SQL Server? Cov EXISTS tus neeg teb xov tooj yog tus neeg teb xov tooj muaj laj thawj uas tso cai rau koj los xyuas seb puas muaj cov lus nug rov qab los ntawm kab. Cov EXISTS tus neeg teb xov tooj rov TRUE yog tias cov lus nug xa rov qab ib lossis ntau kab. Nyob rau hauv no syntax, subquery yog SELECT nqe lus nkaus xwb.
Xav txog qhov no, dab tsi muaj nyob hauv SQL?
SQL ntawv | EXISTS . Cov EXISTS mob hauv SQL ntawv yog siv los xyuas seb qhov tshwm sim ntawm kev sib raug zoo nested query yog khoob (tsis muaj tuples) los yog tsis. Qhov tshwm sim ntawm EXISTS yog tus nqi boolean True lossis False. Nws tuaj yeem siv tau hauv SELECT, UPDATE, INSERT lossis DELETE nqe lus.
Dab tsi yog coalesce hauv SQL?
COALESCE yog ib tug built-in SQLServer Muaj nuj nqi. Siv COALESCE thaum koj xav hloov NULL nrog lwm tus nqi. Nws siv daim ntawv: COALESCE (value1, value2,, valuen) Nws xa rov qab thawj qhov tsis yog NULL los ntawm daim ntawv teev nqi.
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