Ua hauj lwm nrog cov database: Vim li cas cov kev thov

Yuav ua li cas yog lub "thov"? Yuav ua li cas yog feem ntau meant los ntawm cov lus nug nyob rau hauv lub database? Yuav ua li cas yog cov kev thov uas xa mus rau lub database? Nyob rau hauv qhov kev thov rub lub mov ntawm tej yam tej yam kev mob, raws li uas tus database yuav teb thiab muab cov lus qhia ntawm kev txaus siab. Xws li muaj xa tej yam tej yam kev mob / cov ntaub ntawv los ntawm cov uas tus tsim nyog cov ntaub ntawv xaiv thiab kis tau mus rau tus neeg sab / los yog nkag mus rau hauv lub database. Lo lus teb rau lo lus nug "dab tsi yog qhov kwv, qhov kev thov", koj yuav twv yuav raug hu kawm tau los ntawm qhov tsab xov xwm.

Yog vim li cas queries mus rau lub database?

cov lus teb yuav tsum tau muab tsis tau tsuas yog rau lo lus nug ntawm yog dab tsi qhov kev thov, tab sis kuj nyob rau hauv cov nqe lus nug, yog vim li cas yog nug. Lawv yog cov tsim nyog tau txais cov ntaub ntawv uas yog cia nyob rau hauv lub database. Cias muab, lub database queries yog tsim kom tau cov ntaub ntawv uas yuav tsum tau nyob rau hauv tej rooj plaub. Lawv lub hom phiaj yuav ua tau txawv heev: tej zaum nws yuav tseem ceeb kom paub raws li ib tug qhua tuaj noj mov ntawm lub tsev txhab nyiaj rau hauv lub thib peb-tog qhov chaw, los yog kom paub tias raws li ib tug neeg ua hauj lwm intranet, los yog mus rau muab cov ntaub ntawv hais txog lub xeev ntawm cov profile rau ntawm qhov website ntawm qhov kev ua si.

Yuav ua li cas yog lub Cheebtsam ntawm cov lus nug yog?

Peb mus txuas ntxiv mus rau teb rau cov nqe lus nug: Yog vim li cas yog kev thov. SQL yog siv los sau queries. Tas yuav tsum tau tsuas yog ob tug Cheebtsam: xaiv thiab NTAWM. Tab sis sib nrug los ntawm lawv muaj peev xwm yuav siv tau thiab yog siv los ntawm ib tug xov tooj ntawm lwm yam pab pawg uas ntxiv tshiab uas yuav tsum tau rau cov ntaub ntawv xaiv thiab ntsuam xyuas mus tso saib. Cov kev thov yog nyob rau hauv lub database kom txais tau lub feem ntau yam ntaub ntawv ib tug computer: koj yuav tsum nrhiav qhov twg nws yog ib yam dab tsi mus nrhiav. Cov feem nrov tivthaiv qhov chaw yuav tsum tau tom qab lub qhov twg. Qhov twg siv tau, nyob rau hauv thiaj li yuav teem tej kev cai rau cov kev xaiv ntawm cov ntaub ntawv. Piv txwv li, koj yuav hais kom meej lub identification number, hnub yug los yog lwm yam ntaub ntawv uas yog nws thiab hais tias tej zaum yuav qhia hais tias tus neeg.

Lub tsev ib cov lus nug

Tej kev thov muaj ib tug nruj hierarchy ntawm kev tsim kho, uas muaj peev xwm tsis ua txhaum. Rau ib tug yuam kev tej zaum yuav tshwm sim. Cov kev siv yuav tsum piav qhia rau lub hauv paus ntawm ib tug yooj yim cov lus nug nrog peb lub Cheebtsam. Ua ntej muaj yog qho, LOS NTAWM, thiab nyob qhov twg. Tswv yuav ntaus ob qho tib si loj thiab me me ntawv, rau tua yog tsis cuam tshuam. Tab sis raws li cov kev cai ntawm cov lus qhia kom, tag nrho cov tswv yog sau ntawv nrog ib tug capital tsab ntawv thiab yuav tsum tau tej yam kev mob, cov npe ntawm cov ntxhuav thiab khoom nrog ib tug me ntsis. Thiab yog li ntawd ntau yooj yim rau next thaum browsing code. Rov qab mus rau cov code, koj yuav tsum qhia sib nrug, uas yog lub luag hauj lwm rau dab tsi.

Lub tsev ib cov lus nug, raws li ib tug txoj cai, tsis txawv ntawm kev ua hauj lwm nyob rau hauv txawv kev loj hlob tej kev kawm. Yog li ntawd, sawv nyob rau hauv pem hauv ntej ntawm koj ib lo lus nug: "Yog vim li cas yog kev thov rau kev nkag mus» los yog lwm txoj kev loj hlob cheeb tsam, thiab koj yuav nco ntsoov tias cov lus teb muab nyob rau hauv no tsab xov xwm yuav tuaj mus rau lawv tag nrho.

Basic cov ntaub ntawv thov

Lub ntsiab tivthaiv qhov chaw, raws li hais ua ntej lawm, tsuas yog ob tug:

• Xaiv [1 tshaj plaws mus dab tsi koj yuav tsum tau 2, koj yuav tsum 3] - siv rau kom meej li cas cov ntaub ntawv uas yuav tsum tau. Hais tias nws yuav tsum kis los ntawm lub database rau qhov kev pab cuam uas tus neeg siv ua hauj lwm.
• LOS NTAWM [rooj uas cov ntaub ntawv yog npaum li cas] - qhia kom meej qhov yuav tsum tau cov ntaub ntawv no yog me me, thiab tseem yuav tau qhia qhov chaw uas lawv yuav tsum tau npaum li cas. Lub database yog tsis ncaj qha muab cov ntaub ntawv, thiab ib lub rooj uas muaj cov ntaub ntawv. Lub sib txawv ntxhuav yuav muaj tib lub txhua ntawm cov ntaub ntawv kom tsis txhob muaj li no, thiab yog siv los qhia kom meej qhov twg nws yog npaum li cas.

Ntxiv cov ntsiab lus thiab daim ntawv thov pab pawg neeg ua hauj lwm

Koj xav kom koj mus nrhiav tau thiab muab cov ntaub ntawv rau kev siv cov ntaub ntawv twb tau txais ntxiv commands:

• Qhov twg [nrhiav cov nqe lus] - siv rau txheeb tawm qhov tsim nyog ntaub ntawv hais txog cov kev tsim nyog tau txais tej yam kev mob.
• Txwv [number] - yog siv los txo tus naj npawb ntawm kab uas yuav tsum tau noj los ntawm lub rooj.
• GROUP LOS NTAWM [cov lus nug parameter] - yog siv los pab pawg neeg tus tau txais ntaub ntawv los ntawm lub database. Tab sis cov pab pawg neeg yog tsis muaj ntaub ntawv yuav tau nkag, tiam sis tsuas yog nyob rau hauv kev faib ua feem, thiab muaj tib lub hom. Yog xav paub ntxiv kom paub meej koj yuav nrhiav tau tawm los ntawm kev nrhiav ib tug nyias muaj nyias ib tsab xov xwm nyob rau hauv lub GROUP LOS. Group hauj lwm nyob rau hauv queries tsim los txhim kho cov tsos ntawm cov ntaub ntawv thiab nws cov ntau dua readability.
• UNION [thov] yog siv los muab tso rau ib daim ntawv thov subquery. Thaum koj tau txais ib tug ncaj tseem ceeb npaum li cas ntawm cov ntaub ntawv tej zaum yuav xav tau thiab qhov kev xaiv.
• ZOO LI "" yog siv los xyuas seb lub npog ncauj sau raws nkaus Ii qhov kev thov ntawm ib tug me me ntaub ntawv. Yog li ntawd, nrog nws cov kev pab, koj yuav nrhiav tus neeg uas nws cov nyiaj hli vymeryat rau cov nuj nqis.

Piv txwv li

Qhov no nyob rau hauv nws tus kheej yog ib tug problematic kev nkag siab ntawm sau ntawv, tsis muaj ib tug coj ua piv txwv. Tab sis txawm muaj ib zaug xwb piv txwv li tsis tau qhia txhua yam, thiab koj yuav tsum nrhiav ib tug ntau ntawm cov ntaub ntawv ua ntej koj yuav tsum paub txog txhua yam kev kawm muaj SQL developers:

Xaiv lub npe, ProductNumber, ListPrice AS nqe

LOS NTAWM Production.Product

QHOV TWG ProductLine = 'R'

Parse muaj kuv lub saum toj no code. Ua ntej, muaj ib tug xaiv ntawm qhov tsim nyog cov ntaub ntawv: lub npe, khoom tooj thiab cov nplooj ntawv leaflet nqi. Thiab tus nqi ntawv yog tso tawm kom pom ntawm ib tug me ntsis txawv lub npe - ib tug "nqi". Cov ntaub ntawv yog los ntawm cov database "khoom" los ntawm lub rooj "Khoom". Nyob rau hauv kev, qhia kom meej database yog tsis tsim nyog yog hais tias koj ua hauj lwm nrog tsuas yog ib, uas yeej xa ib tug kev thov. Tab sis, yog hais tias ib tug ob peb bases, ces nco ntsoov qhia kom meej, muab qhov tseeb hais tias lub computer tsuas yuav tsis to taub mus rau leej twg koj xa, los yog txawm tsis quav ntsej txog koj daim ntawv thov thiab yuav tsim ib qho yuam kev. Qhov thib peb kab ntawd hais tias qhov tso zis tsis yog tag nrho cov ntaub ntawv, tiam sis tsuas yog ib tug uas mus nyob rau hauv txoj kab ntawm "P" khoom. Uas twb ib luv luv hauv tsab xov xwm, tom qab nyeem ntawv koj to taub tam sim no, yog vim li cas yog nug.