![Dab tsi yog null thiab tsis null hauv SQL? Dab tsi yog null thiab tsis null hauv SQL?](https://i.answers-technology.com/preview/technology-and-computing/14049236-what-is-null-and-not-null-in-sql-j.webp)
Video: Dab tsi yog null thiab tsis null hauv SQL?
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2024 Tus sau: Lynn Donovan | [email protected]. Kawg hloov kho: 2023-12-15 23:47
SQL tsis yog NULL Kev txwv. Cov TSIS NULL txwv enforces ib kab rau TSIS TAU txais NULL tus nqi. Qhov no yuam ib daim teb kom ib txwm muaj tus nqi, uas txhais tau hais tias koj tsis tuaj yeem ntxig cov ntaub ntawv tshiab, lossis hloov kho cov ntaub ntawv yam tsis tau ntxiv tus nqi rau daim teb no.
Raws li, qhov txawv ntawm null thiab tsis null hauv SQL yog dab tsi?
TSIS NULL txhais tau hais tias kab tuaj yeem tsis yog muaj a NULL tus nqi rau tej ntaub ntawv; NULL txhais tau tias NULL yog tus nqi tso cai (txawm tias lub kem muaj qhov txwv txawv teb chaws).
Ib sab saum toj, dab tsi yog null thiab tsis null? NULL thiab tsis NULL yog cov tshev txwv siv los qhia seb puas yuav tsum tso cai nulls lossis tsis yog . Qhov no tuaj yeem muab thaum tsim cov khoom siv database. NULL yog siv los xyuas qhov xwm txheej nrog tus neeg teb xov tooj sib npaug (=). Tsis Null yog siv los xyuas tus nqi uas yog null los yog tsis yog.
Ntawm no, iS NULL hauv SQL?
Cov SQL NULL yog lo lus siv los sawv cev tus nqi uas ploj lawm. A NULL tus nqi nyob rau hauv ib lub rooj yog ib tug nqi nyob rau hauv ib daim teb uas zoo nkaus li dawb paug. Ib lub teb nrog a NULL tus nqi yog ib daim teb uas tsis muaj nqi. Nws tseem ceeb heev kom nkag siab tias a NULL tus nqi txawv dua li tus nqi xoom lossis ib daim teb uas muaj qhov chaw.
Puas yog null nyob qhov twg clause?
YOG NULL & YOG TSIS NULL hauv SQL siv nrog qhov twg lus nyob rau hauv SELECT, UPDATE thiab DELETE cov nqe lus / lus nug kom validate seb puas muaj ib co nqi los yog cov ntaub ntawv tsis muaj nyob rau kab ntawv ntawd. Ib kem nrog NULL tus nqi tsis muaj nqi, nws yog khoob. Syntax rau SQL YOG NULL & YOG TSIS NULL muab hauv qab no.
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