Video: Dab tsi yog qhov kev sib sau ua ke hauv SQL?
2024 Tus sau: Lynn Donovan | [email protected]. Kawg hloov kho: 2023-12-15 23:47
Aggregate functions hauv SQL . Hauv kev tswj hwm database ib aggregate muaj nuj nqi yog a muaj nuj nqi qhov twg qhov tseem ceeb ntawm ntau kab yog pawg ua ke raws li cov tswv yim ntawm qee cov qauv los tsim ib qho txiaj ntsig ntawm ntau lub ntsiab lus tseem ceeb. Ntau yam Aggregate Functions.
Sib piv, dab tsi yog qhov kev ua haujlwm sib sau ua ke hauv SQL nrog piv txwv?
Ib qho kev sib sau ua ke ua ib qho kev suav ntawm ib qho txiaj ntsig, thiab xa rov qab ib zaug tus nqi . Tsuas yog rau COUNT , aggregate functions ignore null values. Kev sib sau ua haujlwm feem ntau siv nrog GROUP BY clause ntawm SELECT nqe lus.
Ib sab saum toj, pes tsawg lub luag haujlwm muaj nyob rau hauv SQL? MySQL txhawb nqa tag nrho tsib (5) ISO tus qauv aggregate functions COUNT, SUM, AVG, MIN thiab MAX.
Kuj paub, qhov twg ntawm cov kev xaiv hauv qab no yog kev sib sau ua haujlwm hauv SQL?
Cov hauv qab no yog cov feem ntau siv SQL aggregate functions: AVG () - rov qab qhov nruab nrab ntawm ib txheej. COUNT () - xa rov qab tus naj npawb ntawm cov khoom hauv ib txheej. MAX() – rov qab qhov siab tshaj plaws tus nqi hauv ib txheej.
Lub luag haujlwm tag nrho nyob qhov twg?
Yog tias SELECT nqe lus suav nrog WHERE clause, tab sis tsis yog GROUP BY clause, ib qho aggregate muaj nuj nqi tsim ib qho nqi rau cov kab sib txuas uas qhov WHERE clause qhia. Thaum twg ib aggregate muaj nuj nqi yog siv nyob rau hauv SELECT nqe lus uas tsis suav nrog GROUP BY clause, nws tsim ib qho nqi.
Pom zoo:
Dab tsi yog qhov sib txawv ntawm kev sib txuas lus taw qhia thiab kev sib txuas tsis raug cai?
Qhov txawv: Kev sib txuas lus taw qhia thiab kev sib txuas tsis muaj kev sib txuas Kev sib txuas lus taw qhia kev sib txuas ua rau muaj kev sib txuas thiab xyuas seb cov lus puas tau txais lossis tsis tau thiab xa rov qab yog tias muaj qhov yuam kev tshwm sim, thaum qhov kev pabcuam tsis sib txuas tsis tau lees tias xa xov
Dab tsi yog qhov sib txawv ntawm qhov kev txiav tawm thiab qhov kev sib cav Ampliative?
Yog hais tias tus arguer ntseeg hais tias qhov tseeb ntawm qhov chaw yeej tsim qhov tseeb ntawm qhov xaus, ces qhov kev sib cav yog deductive. Yog hais tias tus arguer ntseeg hais tias qhov tseeb ntawm qhov chaw muab tsuas yog vim li cas zoo los ntseeg qhov xaus yog tej zaum muaj tseeb, ces qhov kev sib cav yog inductive
Dab tsi yog qhov txawv ntawm kev sib raug zoo unary kev sib raug zoo binary thiab kev sib raug zoo ternary?
Kev sib raug zoo tsis sib xws yog thaum ob tus neeg koom hauv kev sib raug zoo yog tib lub koom haum. Piv txwv li: Cov ncauj lus yuav yog qhov yuav tsum tau ua ua ntej rau lwm yam kev kawm. Kev sib raug zoo ternary yog thaum peb lub koom haum koom nrog kev sib raug zoo
Dab tsi yog qhov sib txawv ntawm kev sib cav inductive thiab kev sib cav deductive?
Cov kev sib cav txiav tawm muaj cov lus xaus tsis txaus ntseeg tias txhua qhov chaw muaj tseeb, tab sis cov lus sib cav sib ceg tsuas muaj qee qhov kev ntsuas ntawm qhov tshwm sim uas qhov kev sib cav muaj tseeb - raws li lub zog ntawm qhov kev sib cav thiab cov pov thawj los txhawb nws
Dab tsi yog qhov sib txawv tseem ceeb ntawm kev sib txuas lus tsis sib xws thiab kev sib txuas lus taw qhia?
1. Hauv kev sib txuas lus tsis muaj kev sib txuas lus tsis tas yuav tsim kev sib txuas ntawm qhov chaw (sender) thiab qhov chaw (receiver). Tab sis nyob rau hauv kev sib txuas-taw qhia kev sib txuas lus kev sib txuas lus yuav tsum tau tsim ua ntej hloov cov ntaub ntawv