Video: Yuav ua li cas koj xyuas seb tus nqi muaj nyob rau hauv ib lub database?
2024 Tus sau: Lynn Donovan | [email protected]. Kawg hloov kho: 2023-12-15 23:47
Rau xyuas seb ib qho tshwj xeeb muaj nuj nqis hauv lub database , koj tsuas yog yuav tsum tau khiav ib txwm SELECT query, nqa ib kab thiab saib seb dab tsi tau nqa. Ntawm no peb tab tom xaiv kab uas phim peb cov qauv, tom qab ntawd nqa nws thiab tom qab ntawd tshuaj xyuas seb txhua yam tau xaiv los tsis tau.
Ntawm no, koj yuav ua li cas xyuas seb tus nqi puas nyob hauv lub rooj hauv SQL?
Rau kuaj seb ib row muaj nyob hauv MySQL rooj los tsis, siv muaj nyob mob. Cov muaj nyob mob tuaj yeem siv nrog subquery. Nws rov qab muaj tseeb thaum kab muaj nyob hauv rooj , txwv tsis pub dag rov qab. Qhov tseeb yog sawv cev rau hauv daim ntawv ntawm 1 thiab cuav yog sawv cev li 0.
Tsis tas li ntawd, coalesce hauv SQL yog dab tsi? 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.
Yuav ua li cas yog tias tsis muaj nyob hauv SQL?
los ntawm suresh. Cov SQL tsis muaj Tus neeg teb xov tooj yuav ua qhov sib txawv rau EXISTS Tus neeg ua haujlwm. Nws yog siv los txwv tus naj npawb ntawm kab rov qab los ntawm SELECT Statement. Cov Tsis muaj nyob hauv SQL Server yuav tshawb xyuas Subquery rau kab hav zoov, thiab yog muaj tsis muaj kab ces nws yuav rov TRUE, txwv tsis pub FALSE.
Puas yog 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.
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