Cov txheej txheem:
Video: Cov lus nug nyob qhov twg?
2024 Tus sau: Lynn Donovan | [email protected]. Kawg hloov kho: 2023-12-15 23:47
Dab tsi yog a Subquery ? A subquery , kuj hu ua a nested lus nug los yog subselect, yog a SELECT cov lus nug embedded nyob rau hauv qhov twg los yog HAVING clause ntawm lwm SQL nug . Cov ntaub ntawv xa rov qab los ntawm lub subquery yog siv los ntawm cov nqe lus sab nraud ib yam li tus nqi literal yuav siv.
Hais txog qhov no, dab tsi yog cov lus nug nested?
A Subquery los yog Cov lus nug sab hauv los yog a Nested query yog a nug hauv lwm SQL nug thiab embedded nyob rau hauv lub WHERE clause. A subquery yog siv los xa cov ntaub ntawv uas yuav siv rau hauv lub ntsiab nug raws li ib qho xwm txheej kom txwv tsis pub cov ntaub ntawv yuav tsum tau rov qab.
Tsis tas li, pes tsawg Subqueries tuaj yeem ua zes hauv SQL? A subquery tuaj yeem ua zes nyob rau hauv qhov twg los yog HAVING clause ntawm ib qho kev xaiv sab nrauv, INSERT, Hloov Kho, lossis DELETE nqe lus, lossis hauv lwm qhov subquery . Txog 32 qib ntawm ua zes muaj peev xwm, txawm hais tias qhov txwv txawv raws li muaj kev nco thiab qhov nyuaj ntawm lwm cov kab lus hauv cov lus nug.
Tsis tas li ntawd, koj sau cov lus nug SQL nested li cas?
SQL Subqueries
- Ib subquery tuaj yeem tshwm sim hauv:
- Lub subquery tuaj yeem ua zes hauv SELECT, INSERT, UPDATE, lossis DELETE cov lus lossis hauv lwm cov lus nug.
- Ib qho lus nug feem ntau yog ntxiv nyob rau hauv QHOV CHAW UA HAUJ LWM ntawm lwm nqe lus SQL SELECT.
- Koj tuaj yeem siv cov neeg ua haujlwm sib piv, xws li >, <, lossis =.
Cov lus nug twg yog thawj zaug hauv ib qho nested subquery?
Nrog rau ib txwm nested subquery, cov lus nug sab hauv SELECT khiav ua ntej thiab ua tiav ib zaug, rov qab los tus nqi siv los ntawm cov lus nug tseem ceeb. Ib qho kev sib txheeb subquery, txawm li cas los xij, ua tiav ib zaug rau txhua tus neeg sib tw kab uas xav tau los ntawm cov lus nug sab nraud. Hauv lwm lo lus, cov lus nug sab hauv yog tsav los ntawm cov lus nug sab nraud.
Pom zoo:
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