Qhov Tseem Ceeb Qhov txawv - qhov twg vs muaj cov lus hauv SQL
Cov ntaub ntawv tseem ceeb rau txhua lub koom haum. Yog li ntawd, nws yog ib qho tsim nyog yuav tsum khaws cov ntaub ntawv hauv ib txoj hauv kev los muab lawv. Cov ntaub ntawv khaws cia hauv cov ntxhuav. Ib tug database muaj ib tug sau ntawm cov ntxhuav. Ib hom databases feem ntau yog relational databases. Hauv cov ntaub ntawv sib raug zoo, cov ntxhuav muaj feem cuam tshuam rau ib leeg. Piv txwv li, cov neeg siv khoom sib txuas nrog cov lus txiav txim. A Relational Database Management System (RDBMS) yog ib qho kev tswj xyuas cov ntaub ntawv uas yog raws li cov qauv kev sib raug zoo. Nws yog siv los tswj Relational Databases. Qee qhov piv txwv ntawm RDBMS yog MySQL, MSSQL, thiab Oracle. Structured Query Language (SQL) yog hom lus siv los tswj thiab khaws cov ntaub ntawv hauv cov ntaub ntawv sib txheeb. Muaj ntau cov lus hauv SQL los ua cov haujlwm sib txawv. Ob tug ntawm lawv nyob qhov twg thiab muaj. Kab lus no tham txog qhov sib txawv ntawm qhov twg thiab muaj kab lus hauv SQL. Qhov sib txawv tseem ceeb ntawm qhov twg thiab muaj clause hauv SQL yog qhov twg clause siv los lim cov ntaub ntawv ua ntej kev sib sau ua ke lossis kev sib sau ua ke tshwm sim thaum muaj clause yog siv los lim cov ntaub ntawv tom qab pab pawg, lossis kev sib sau tshwm sim.
Dab tsi yog qhov clause hauv SQL?
Nws pab khaws, hloov kho lossis tshem tawm cov ntaub ntawv tshwj xeeb ntawm lub rooj raws li qhov xwm txheej tau muab. Tus programmer siv tau qhov twg clause txwv thiab nqa tsuas yog cov ntaub ntawv xav tau. Cov lus nug tsuas yog siv rau ntawm cov ntaub ntawv uas qhov xwm txheej tau teev tseg los ntawm qhov twg clause muaj tseeb. Nws tuaj yeem siv nrog xaiv, hloov kho thiab tshem tawm.
Saib cov menyuam kawm ntawv hauv qab no,
Xaiv lub npe thiab hnub nyoog ntawm tus menyuam kawm ntawv uas nws tus tub kawm ntawv_id sib npaug rau 3, cov lus nug SQL hauv qab no tuaj yeem siv.
xaiv lub npe, hnub nyoog ntawm tus menyuam kawm qhov twg student_id=3;
Nws tseem tuaj yeem siv cov tswv lag luam xws li tsis sib npaug (!=), ntau dua (>), tsawg dua (=), tsawg dua lossis sib npaug (<=). Txhawm rau xaiv tus tub ntxhais kawm_id thiab lub npe uas nws hnub nyoog tsis sib npaug li 15, cov lus nug SQL hauv qab no tuaj yeem siv.
xaiv student_id, npe los ntawm tus menyuam kawm ntawv hnub nyoog!=15;
Yuav hloov lub hnub nyoog ntawm tus menyuam kawm ntawv 2 mus rau 13, cov lus nug hauv qab no tuaj yeem siv.
hloov tshiab tub ntxhais kawm teem hnub nyoog=13 qhov twg id=3;
Yuav rho tawm cov ntaub ntawv uas tus tub kawm ntawv_id yog 4, cov lus nug hauv qab no tuaj yeem siv.
delete los ntawm tub kawm qhov twg student_id=4;
Tus thiab, lossis tus neeg siv tuaj yeem siv los ua ke ntau yam xwm txheej.
xaiv lub npe ntawm tus menyuam kawm qhov twg student_id=1 thiab hnub nyoog=15; Cov lus nug yuav muab lub npe Ann.
Cov no yog qee qhov piv txwv ntawm qhov twg clause hauv SQL. Yog hais tias muaj ib pab pawg los ntawm clause, qhov twg clause tshwm ua ntej ntawd.
Dab tsi muaj clause hauv SQL?
Muaj cov haujlwm muab los ntawm SQL lus los ua kev suav yooj yim. Lawv hu ua aggregation functions. Tus min () yog siv los nrhiav tus nqi tsawg tshaj plaws ntawm kab xaiv. Qhov max () yog siv los nrhiav tus nqi siab tshaj plaws ntawm kab xaiv. Lub avg () yog siv los nrhiav qhov nruab nrab hauv kab thiab sum () yog siv los nrhiav tag nrho ntawm kab. Nov yog qee qhov piv txwv ntawm kev sib sau ua ke. Xa mus rau lub rooj txiav txim hauv qab no,
Tus programmer tuaj yeem sau cov lus nug SQL raws li hauv qab no txhawm rau khaws cov neeg siv khoom uas nws qhov nyiaj tshuav ntau dua 2000.
xaivlos ntawm pawg neeg xaj los ntawm cov neeg siv khoom muaj cov lej (sib npaug) > 2000.
Qhov no yuav luam tawm cov neeg siv khoom cov ntaub ntawv uas nws cov ntsiab lus ntawm qhov nyiaj tshuav yog loj dua 2000. Nws yuav luam cov ntaub ntawv ntawm cov neeg siv khoom Ann thiab Alex.
Cov lus muaj yog siv los khaws cov txiaj ntsig rau cov pab pawg uas txaus siab rau qee yam xwm txheej. Yog li ntawd, cov pab pawg neeg uas poob mus rau qhov muab qhov xwm txheej yuav tshwm sim raws li qhov tshwm sim ntawm qhov no. Qhov muaj clause tshwm tom qab pawg-los ntawm kab lus. Yog hais tias pawg-los ntawm cov lus tsis nyob, ces cov lus muaj yuav ua haujlwm zoo ib yam li qhov clause.
Dab tsi yog qhov zoo sib xws ntawm qhov twg thiab muaj cov lus hauv SQL?
- Ob leeg yog clauses hauv Structured Query Language.
- Ob leeg siv tau los lim cov ntaub ntawv.
Qhov txawv ntawm qhov twg thiab muaj cov lus hauv SQL?
qhov twg vs muaj cov lus hauv SQL |
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| Qhov twg yog SQL clause uas yog siv los qhia qhov xwm txheej thaum nqa cov ntaub ntawv los ntawm ib lub rooj lossis los ntawm kev koom nrog ntau lub rooj. | Qhov muaj yog SQL clause uas qhia meej tias SQL xaiv nqe lus yuav tsum tsuas yog xa rov qab cov kab uas cov nqi sib sau ua tau raws li cov xwm txheej tau teev tseg. |
| Lub Hom Phiaj | |
| Qhov twg clause siv los lim kab. | Cov lus muaj yog siv los lim cov pab pawg. |
| Kev sib sau | |
| Qhov twg clause tsis tuaj yeem siv nrog kev sib sau ua ke tshwj tsis yog nws nyob hauv cov lus nug uas muaj nyob rau hauv Muaj cov lus. | Kev sib sau ua haujlwm tuaj yeem siv nrog cov lus muaj. |
| Filting Method | |
| Qhov twg clause coj tus cwj pwm ua ntej lim. | Tus muaj nqe lus coj tus cwj pwm zoo li tus ncej lim. |
| Group Los ntawm Clause Order | |
| Qhov twg clause siv ua ntej Pawg By clause. | Tus muaj clause yog siv tom qab Pab Pawg Los ntawm clause. |
| Us nrog | |
| Qhov twg clause tuaj yeem siv nrog xaiv, hloov kho thiab tshem tawm. | Qhov muaj nqe lus tsuas yog siv nrog xaiv. |
Summary – qhov twg vs muaj clause hauv SQL
Cov Lus Nug Lus Nug (SQL) yog cov lus siv hauv cov ntaub ntawv sib txheeb. Qhov twg muaj yog ob nqe lus hauv SQL. Kab lus no tau tham txog qhov sib txawv ntawm qhov twg thiab muaj kab lus. Qhov txawv ntawm qhov twg thiab muaj clause hauv SQL yog qhov twg siv los lim cov ntaub ntawv ua ntej kev sib koom ua ke lossis kev sib sau ua ke tshwm sim thaum siv los lim cov ntaub ntawv tom qab kev sib sau ua ke, lossis kev sib sau ua ke tshwm sim.
