Video: Dab tsi yog sab laug hauv MySQL?
2024 Tus sau: Lynn Donovan | [email protected]. Kawg hloov kho: 2023-12-15 23:47
LEEJ TWG () ua haujlwm
MySQL LEFT () xa rov qab ib tus lej ntawm cov cim los ntawm sab laug ntawm txoj hlua. Ob tus naj npawb thiab txoj hlua tau muab los ua kev sib cav ntawm txoj haujlwm
Xav txog qhov no, dab tsi yog sab laug koom nrog MySQL?
MySQL LEFT JOIN . Cov LEFT JOIN yog siv los xa cov ntaub ntawv los ntawm ntau lub rooj. Hauv particular, " LEEJ TWG " ib feem txhais tau tias tag nrho cov kab los ntawm sab laug lub rooj yuav raug xa rov qab, txawm tias tsis muaj kab sib txuam hauv lub rooj zoo. Qhov no tuaj yeem ua rau NULL qhov tseem ceeb tshwm sim hauv txhua kab rov qab los ntawm lub rooj sab xis.
Ib yam li ntawd, siv sab laug thiab txoj cai li cas hauv SQL? 8 T-SQL String Functions
- LEEJ TWG. Koj siv LEFT muaj nuj nqi kom xa rov qab cov lej cim ntawm ib txoj hlua sab laug.
- YOG. Txoj haujlwm RIGHT rov qab ib tus lej ntawm cov cim los ntawm txoj hlua sab xis.
- LTRIM. LTRIM muaj nuj nqi tshem tawm cov qhov khoob ntawm txoj hlua.
- RTRIM.
- SUBSTRING.
- Hloov.
- KHOOM.
Ua li no, dab tsi yog muaj nyob rau hauv MySQL?
Cov MUAJ clause yog siv nyob rau hauv SELECT nqe lus qhia kom meej lim tej yam kev mob rau ib pawg ntawm kab los yog aggregates. Cov MUAJ clause feem ntau siv nrog GROUP BY clause los lim cov pab pawg raws li qhov xwm txheej. Yog tias GROUP BY clause raug tshem tawm, tus MUAJ clause coj zoo li WHERE clause.
Qhov twg yog lub rooj sab laug nyob rau sab laug koom?
Cov sab laug rooj yog cov rooj uas yog nyob rau hauv FROM clause, los yog sab laug ntawm koom condition, cov koom clause ntawm no. Thiab ib txoj cai rooj yog nyob rau sab xis ntawm lub koom lus. Thaum peb tham txog ib sab laug sab nrauv koom , qhov peb tab tom hais yog, nqa tag nrho cov kab los ntawm sab laug rooj , thiab koom lawv mus lem sab xis rooj.
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