Video: Qhov twg tsis nyob VS qhov twg tsis muaj?
2024 Tus sau: Lynn Donovan | [email protected]. Kawg hloov kho: 2023-12-15 23:47
Qhov tseem ceeb tshaj plaws yuav tsum nco ntsoov txog TSIS MUAJ thiab TSIS TAU IN yog tias, tsis zoo li EXISTS thiab IN, lawv yog tsis yog sib npaug hauv txhua kis. Tshwj xeeb, thaum NULLs koom nrog lawv yuav rov qab tau cov txiaj ntsig sib txawv. Yuav kom meej meej tag nrho, thaum lub subquery rov qab txawm tias ib qho null, TSIS TAU IN yuav tsis yog phim txhua kab.
Ntawm no, qhov twg Tsis Muaj vs qhov twg tsis nyob?
Qhov tseem ceeb tshaj plaws yuav tsum nco ntsoov txog TSIS MUAJ thiab TSIS IN yog tias, tsis zoo li EXISTS thiab IN, lawv yog tsis yog sib npaug hauv txhua kis. Tshwj xeeb, thaum NULLs koom nrog lawv yuav rov qab tau cov txiaj ntsig sib txawv. Yuav kom meej meej tag nrho, thaum lub subquery rov qab txawm tias ib qho null, TSIS IN yuav tsis yog phim txhua kab.
Tsis tas li ntawd, nyob qhov twg thiab tsis muaj nyob hauv SQL? Lub hom phiaj ntawm cov SQL ntawv “ Muaj "thiab" Tsis Muaj ” tus neeg teb xov tooj yog txhawm rau txheeb xyuas qhov muaj nyob ntawm cov ntaub ntawv hauv cov lus nug. Thaum twg SQL muaj yog siv nyob rau qhov twg clause, nws ntsuam xyuas lub hav zoov ntawm kab nyob rau hauv ib tug subquery. Yog hais tias qhov subquery muaj ib kab ces nws rov qab rau TRUE.
Ib yam li ntawd, tib neeg nug, qhov txawv ntawm qhov tsis nyob hauv thiab tsis muaj?
Raws li txoj cai ntawm tus ntiv tes xoo, kuv nyiam tsis muaj nyob raws li nws npog ntau qhov xwm txheej ntau dua li tsis yog hauv. Tsis muaj nyob tuaj yeem siv rau txhua qhov xwm txheej uas tsis yog nyob rau hauv yog siv rau, tab sis tsis yog rov qab. Yuav muaj kev ua tau zoo sib txawv , nrog muaj nyob ua sai dua. Qhov tseem ceeb tshaj plaws txawv yog kev tuav ntawm nulls.
Puas yog null vs tsis muaj?
Cov NULL yog xam thiab xa rov qab los ntawm lub TSIS TAU IN hais kom ua tus nqi. Lub SQL TSIS MUAJ hais kom ua yog siv los xyuas seb qhov muaj nuj nqis tshwj xeeb hauv cov lus nug tau muab. Lub subquery yuav tsis yog xa cov ntaub ntawv; nws rov TRUE lossis FALSE qhov tseem ceeb nyob ntawm qhov kev tshawb nrhiav qhov tseem ceeb muaj nyob.
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