Lus nug cov lus nug ntawm lub database
Ib cov lus nug database rho tawm cov ntaub ntawv los ntawm lub database thiab nws sau rau hauv daim ntawv nyeem. Ib cov lus nug yuav tsum tau sau nyob rau ntawm hom lus tus database yuav tsum tau-feem ntau, cov lus ntawd yog SQL .
Piv txwv, thaum koj xav tau cov ntaub ntawv los ntawm cov ntaub ntawv database, koj siv cov lus nug los thov cov lus qhia koj xav tau. Kab tias koj muaj ib tug neeg ua hauj lwm cov lus, thiab koj xav taug qab cov naj npawb kev muag khoom muag. Koj tuaj yeem nug koj tus database rau tus neeg ua haujlwm uas kaw qhov khoom muag siab tshaj plaws hauv ib lub sijhawm.
SQL SELECT Statement
Ib cov lus nug database yuav tsum ua raws li cov lus nug hom ntawv uas yuav tsum tau los ntawm tus database. Qhov hom tshaj plaws yog Lus Askiv Lus (Structured Query Language) (SQL) cov qauv lus nug uas siv los ntawm ntau cov kev tswj xyuas cov tshuab. SQL yog ib hom lus muaj zog ntawm cov queries siab heev.
SQL siv ib qho lus xaiv los xaiv cov ntaub ntawv.
Xav txog ib qho piv txwv raws li ntawm Northwind database uas nquag ships nrog cov khoom database raws li tus nyeem.
Ntawm no yog ib qho excerpt los ntawm lub database Cov neeg ua hauj lwm lub rooj:
| Cov neeg ua haujlwm | LastName | FirstName | Lub npe | Chaw nyob | Nroog | Thaj av | |
|---|---|---|---|---|---|---|---|
| 1 | Davolio | Nancy | Tus Muag Sawv Cev | 507 - 20th Ave. E. | Seattle | WA | |
| 2 | Fuller | Andrew |
| 908 W. Txoj Kev Suav | Tacoma | WA | |
| 3 | Leverling | Janet | Tus Muag Sawv Cev | 722 Moss Bay Blvd. | Kirkland | WA |
Xa cov neeg ua haujlwm npe thiab lub npe ntawm tus neeg ua haujlwm, cov ntaub ntawv SELECT yuav pom zoo li qhov no:
NPE SAU NPE, Lub Npe, Lub Npe Ntawm Cov Neeg Ua Hauj Lwm;
Nws yuav rov qab los:
| FirstName | LastName | Lub npe |
|---|---|---|
| Nancy | Davolio | Tus Muag Sawv Cev |
| Andrew | Fuller | Tus Lwm Thawj, Kev Muag Khoom |
| Janet | Leverling | Tus Muag Sawv Cev |
Los ua kom zoo dua ntxiv, koj yuav ntxiv qhov chaw txiav txim:
Xaiv npe FirstName, LastName FROM Employees
QHOV CHAW City = 'Tacoma';
Nws rov qab los ntawm Lub Npe thiab Lub Npe ntawm txhua tus neeg ua haujlwm uas yog los ntawm Tacoma:
| FirstName | LastName |
|---|---|
| Andrew | Fuller |
Nco ntsoov tias SQL rov qab cov ntaub ntawv nyob rau hauv ib kab / kem daim ntawv uas zoo ib yam li Microsoft Excel, ua kom pom yooj yim thiab ua haujlwm nrog. Lwm cov lus nug lus yuav xa cov ntaub ntawv raws li graph lossis daim duab qhia.
Lub hwj chim ntawm cov lus nug
Ib cov ntaub ntawv muaj peev xwm nthuav qhia txoj haujlwm thiab kev ua ub no, tiam sis lub hwj chim no tsuas yog siv los ntawm kev siv cov lus nug. Ib txoj kab database muaj ntau lub rooj khaws cia ua ib qho myriad ntawm cov ntaub ntawv. Ib cov lus nug pub koj lim nws mus rau hauv ib lub rooj tsuas yog koj tuaj yeem yooj yim txheeb xyuas nws.
Cov lus nug tseem tuaj yeem ua cov lej ntawm koj cov ntaub ntawv lossis siv cov ntaub ntawv tswj xyuas cov haujlwm. Koj tuaj yeem saib xyuas cov tshiab rau koj cov ntaub ntawv ua ntej lawv ua cov ntaub ntawv.