He SQL: whakaahuatanga, wetereo, tauira

SQL Ko te reo paerewa mō te mahi ki te pātengi raraunga tūhonohono. He ia i tona tikanga he huinga o taputapu kaha mō te raweke raraunga rongoa i roto i nga tepu.

Kore, ko te kaha ki raraunga rōpū i roto i to ratou tauira i runga i te pūtake motuhake ko tetahi taua taputapu. tauākī SQL ka haere ki te HEA taea rarangi ki a koe te tautuhi i te tikanga o te tauira i whakarōpūtia te raraunga i roto i te tahi mau ara.

SQL he Tawhā: Whakaahuatanga

Tuatahi o te katoa ko reira ia tapao e he kōwhiringa tenei tawhā, me te whakamahia anake i roto i te taha ki te GROUP BY kōwhiringa. Ka rite ki mahara koe, whakamahia te GROUP BY rara te ka i roto i te Tīpakohia whakamahi te mahi töpü me nga hua o to ratou tātaitanga ki te tiki ki etahi rōpū. Ki te taea te HEA ki a koe whakapūtā tikanga kōwhiringa i mua whakarōpū te raraunga kei te, te ka kei tikanga e pā ana ki ngā raraunga kua tika i roto i te rōpū ratou. Hoki pai te māramatanga, kia haere a roto i te tauira ki te kaupapa aroaro i roto i te ahua i raro.

Ko te tauira nui o hoatu he whakaahuatanga SQL tenei. He he tepu ki te rārangi o ngā ingoa o ngā hua, kamupene whakaputa ratou, me etahi atu mara. Te uiui i roto i te kokonga matau-ringa o runga, e tamata tatou ki te tiki mōhiohio i runga i te tini o ngā momo o ngā hua ia kamupene manufactures, me te hua e hiahia ana tatou ki te whakaatu anake te hunga kamupene e hua ake atu i te 2 ngā tūemi. GROUP BY kōwhiringa i hanga te o toru ngā rōpū e ōrite ana ki te ingoa o ngā kamupene, ia o nei kua kiia te maha o ngā hua (rarangi). Ko te whakatakoto i tō huru wehenga ka kotahi rōpū i te tauira hua, mai kore e makona i te reira te huru. Ka rite ki te hua, whiwhi tatou e rua ngā rōpū e ōrite ana ki te maha o te production kamupene 5 me te 3.

kia whakatika i te pātai rite ki te aha te whakamahi i te ka, ki te mea i reira te mea he SQL HEA. Ki te whakamahia e matou te HEA, e kua titiro ia ki te katoa te maha o nga rarangi i roto i te tepu, nui atu i roto i ngā rōpū, a kihai i pai te huru hanga tikanga i roto i tenei take. Heoi, tino maha e taea e ratou te ora tahi i roto i te uiui kotahi.

I roto i te tauira i runga ake ka taea e kite tatou pehea i mua i reira ko te kōwhiringa o raraunga i runga i mo o te kaimahi i whakaritea i roto i te tawhā HEA, a ka whakarōpū i roto i te GROUP BY hua e ato'a whakamatauria e te moni o te utu mo ia kaimahi.

Tawhā SQL ka: tauira, wetereo

A feruri i te tahi o nga āhuatanga he wetereo SQL. Whakaahuatanga o tenei tawhā Ko te tino māmā. Tuatahi, rite kua whakahuatia, kua whakamahia te reira anake i roto i te taha ki te GROUP tawhā BY ka tohu tonu muri iho me te aroaro o ORDER BY, ki te reira i roto i te tono. He mārama te reira, mai ka whakatau nga tikanga mo te raraunga kua whakarōpūtia. Tuarua, i roto i te huru o tenei tawhā taea te whakamahi anake ngā mahi töpü me nga mara i tabulahia i roto i te tawhā GROUP BY. nga tikanga katoa i roto i tenei tawhā tohu te ara ano e rite ana i roto i te take o HEA.

mutunga

Ka rite ki a koutou e taea te kite, kahore faingataa i roto i tenei tauākī kore mahi. Semantically, Kei te whakamahia te reira i roto i te ara ano rite te HEA. He mea nui ki te matau e te whakamahi i HEA i runga i ngā raraunga katoa tīpakohia me ka - faatura ki te rōpū tautuhia i roto i te GROUP tawhā BY anake ki. aroaro matou he whakaahuatanga whānui mō te he SQL, i te mea e tau mo te whāwhā māia.