Dab tsi yog qhov tshwj xeeb txwv hauv Oracle?
Dab tsi yog qhov tshwj xeeb txwv hauv Oracle?

Video: Dab tsi yog qhov tshwj xeeb txwv hauv Oracle?

Video: Dab tsi yog qhov tshwj xeeb txwv hauv Oracle?
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A tshwj xeeb txwv yog ib daim teb los yog ua ke ntawm cov teb uas tshwj xeeb txhais cov ntaub ntawv. Qee qhov chaw tuaj yeem muaj qhov tsis muaj nuj nqis ntev npaum li qhov sib xyaw ua ke ntawm qhov tseem ceeb tshwj xeeb.

Yog li, dab tsi yog qhov tshwj xeeb tseem ceeb hauv Oracle?

Oracle tshwj xeeb txwv syntax A tshwj xeeb txwv yog kev ncaj ncees txwv uas ua kom cov ntaub ntawv khaws cia rau hauv ib kem, lossis ib pawg ntawm kab, yog tshwj xeeb ntawm cov kab hauv ib lub rooj. Qhov no tshwj xeeb txwv qhia tias qhov tseem ceeb hauv column_name yog tshwj xeeb hla tag nrho lub rooj.

Dab tsi yog qhov ua haujlwm ntawm qhov tshwj xeeb txwv? Piav: Lub lub hom phiaj ntawm qhov tshwj xeeb clause yog los xyuas kom meej tias tsis muaj ob qhov tseem ceeb hauv tib tus cwj pwm zoo ib yam. Cov yuam sij tseem ceeb yog tshwj xeeb los ntawm lub neej ntawd.

Cov lus nug tseem yog, dab tsi yog qhov txwv hauv Oracle?

Siv a txwv los txhais ib qho kev ncaj ncees txwv --ib txoj cai uas txwv cov nqi hauv ib lub database. Oracle Database cia koj tsim rau rau hom kev txwv thiab cia koj tshaj tawm lawv ob txoj kev. Ib qho tseem ceeb txwv combines ib tug TSIS NULL txwv thiab ib qho tshwj xeeb txwv nyob rau hauv ib qho kev tshaj tawm.

Dab tsi yog qhov txawv ntawm qhov ntsuas tshwj xeeb thiab qhov tshwj xeeb txwv hauv Oracle?

Cim index yog rau kev ua haujlwm. Txawm li cas los tshwj xeeb txwv thiab cim indexes ob leeg pab nrog tshwj xeeb , lawv lub hom phiaj txawv. A tshwj xeeb txwv yog txhais los tswj cov ntaub ntawv ncaj ncees. A tshwj xeeb txwv tej zaum yuav tsim ib cim index implicitly, tab sis nws tsis vam los yog xav tau qhov ntsuas tswj cov ntaub ntawv ncaj ncees.

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