Video: Nws puas tuaj yeem yog null SQL?
2024 Tus sau: Lynn Donovan | [email protected]. Kawg hloov kho: 2023-12-15 23:47
Null yog ib tug tshwj xeeb logical nqi nyob rau hauv SQL ntawv . A NULL tus nqi ua tau raug teem. NULL yog qhov tsis muaj nqi, yog li nws ua tau raug xa mus rau TEXT kab, INTEGER kab los yog lwm yam datatype. Ib kem ua tau tsis muaj NULLs tsuas yog tias nws tau tshaj tawm tias TSIS TAU NULL (saib ALTER TABLE).
Hais txog qhov no, tus lej puas tuaj yeem yog null?
Tsis muaj " NULL "rau tus lej . Cov NULL yog tus nqi tshwj xeeb vim nws tsis yog tus nqi pointer siv tau. Li no, peb ua tau siv nws ua ib tug semaphore los qhia tias "qhov no pointer ua tsis taw tes rau dab tsi (tsim nyog)". tus lej yog siv tau, tshwj tsis yog tias koj qhov chaws xav tias lwm yam.
Tsis tas li, peb tuaj yeem hloov kho NULL tus nqi hauv SQL? SQL NULL Values . Yog tias ib kab hauv ib lub rooj yog xaiv tau, peb ua tau ntxig ib cov ntaub ntawv tshiab los yog hloov tshiab ib cov ntaub ntawv uas twb muaj lawm yam tsis tau ntxiv a tus nqi mus rau kab ntawv no. Qhov no txhais tau hais tias lub teb yuav yuav cawm tau nrog ib NULL tus nqi . NULL cov nqi raug kho txawv ntawm lwm yam tus nqi.
Raws li, puas yog null hauv kuv SQL?
Hauv MySQL , a NULL tus nqi txhais tau tias tsis paub. Yog koj sib piv a NULL nqi nrog lwm tus NULL tus nqi lossis lwm yam nqi, qhov tshwm sim yog NULL vim tus nqi ntawm txhua tus NULL tus nqi tsis paub. Feem ntau, koj siv lub NULL tus nqi qhia tias cov ntaub ntawv ploj lawm, tsis paub, lossis tsis siv tau.
Tsis yog NULL rov qab NULL qhov tseem ceeb?
Cov TSIS YOG NULL mob yog siv nyob rau hauv SQL mus kuaj rau a tsis yog - NULL tus nqi. Nws rov qab los TRUE if a tsis yog - NULL tus nqi pom, txwv tsis pub nws rov qab los FALSE. Nws tuaj yeem siv tau hauv SELECT, INSERT, UPDATE, lossis DELETE nqe lus.
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