Gwiritsani ntchito SQL COUNT kuti mubwererenso deta yambiri
Zopangira mafunso ndi gawo lofunikira la Structured Query Language (SQL). Icho chimasungira deta molingana ndi zofunikira zina kuchokera ku deta yolumikizana. Mukhoza kugwiritsa ntchito mafunso a SQL - kuphatikizapo COUNT () ntchito - kupeza zinthu zamtundu uliwonse kuchokera ku deta.
Ntchito ya SQL COUNT () ikugwira ntchito makamaka chifukwa imakulolani kuwerengetsa zolemba zanu zochokera kumagwiritsidwe ntchito. Mukhoza kuzigwiritsa ntchito kuti muwerenge zolembedwa zonse patebulo, kuwerengera zamtengo wapatali m'ndandanda, kapena kuwerengera nthawi zolemba zomwe zikuchitika zomwe zimakwaniritsa zofunikira zina.
Nkhaniyi ikuwunika mwachidule pa zochitika izi.
Zitsanzozi zimachokera ku deta ya Northwind yomwe imagwiritsidwa ntchito kawirikawiri, yomwe nthawi zambiri imatumizidwa ndi zinthu zogulitsira malonda kuti zigwiritsidwe ntchito ngati phunziro.
Pano pali ndondomeko yochokera ku ndondomeko ya Masitolo:
| ProductID | Zamalonda | SupplierID | ChiwerengeroPerUnit | Mtengo wagawo | UnitsIntock |
|---|---|---|---|---|---|
| 1 | Chai | 1 | Mabokosi 10 x matumba 20 | 18.00 | 39 |
| 2 | Kusintha | 1 | 24 - 12 oz mabotolo | 19.00 | 17 |
| 3 | Sirave Yowoneka | 1 | 12 - 550 ml mabotolo | 10.00 | 13 |
| 4 | Mtsinje wa Anton wa Cajun Seasoning | 2 | 48 - 6 oz mitsuko | 22.00 | 53 |
| 5 | Gumbo Mix ya Chef Anton | 2 | 36 mabokosi | 21.35 | 0 |
| 6 | Agogo a Boysenberry Akufalikira | 3 | 12 - 8 oz mitsuko | 25.00 | 120 |
| 7 | Amalonda a Amal Bob's Peanic Zouma | 3 | 12 - 1 lb pkgs. | 30.00 | 15 |
Kuwerenga Zolemba M'ndandanda
Funso lofunikira kwambiri ndilowerengera chiwerengero cha marekodi omwe ali mu tebulo. Ngati mukufuna kudziwa chiwerengero cha zinthu zomwe zilipo patebulo lamagetsi, gwiritsani ntchito funso ili:
SANKANI COUNT (*)
Kuchokera ku mankhwala;
Funso ili limabwereza mzere wa mizere yomwe ili patebulo. Mu chitsanzo ichi, ndi 7.
Kuwerengera Makhalidwe Owapadera Pachilumba
Mungagwiritsenso ntchito COUNT ntchito kuti muzindikire chiwerengero chazidziwikiratu pazomwe zilipo. Mu chitsanzo, ngati mukufuna kudziwa chiwerengero cha ogulitsa ogulitsa awo omwe akupezeka mu dipatimenti yopanga, mungathe kuchita izi pogwiritsa ntchito funso ili:
SANKANI COUNT (DISTINCT SupplierID)
Kuchokera ku mankhwala;
Funso ili limabweretsanso chiwerengero cha machitidwe osiyana omwe akupezeka m'ndandanda ya SupplierID. Pankhaniyi, yankho liri 3, loyimira 1, 2, ndi 3.
Kuwerengera Zolemba Zosakaniza Zofanana
Gwiritsani ntchito COUNT () kugwira ntchito ndi WHERE ndime kuti mudziwe chiwerengero cha zolemba zomwe zikugwirizana ndi zofunikira zina. Mwachitsanzo, tiyerekeze kuti woyang'anira dipatimentiyo akufuna kuti adziŵe za chiwerengero cha masitolo. Funso lotsatira likuwunikira chiwerengero cha mizere yoimira UnitsMagulu osachepera 50 mayunitsi:
SANKANI COUNT (*)
Kuchokera ku mankhwala
PAMENE UnitsIntock <50;
Pankhaniyi, funsoli likanabweretsanso mtengo wa 4, woimira Chai, Chang, Aniseed Syrup, ndi Peyala Bob's Organic Dry Pears.
Chigamulo cha COUNT () chikhoza kukhala chofunika kwambiri kwa olamulira azinthu omwe amayesetsa kufotokozera mwachidule deta kuti akwaniritse zofunikira za bizinesi. Ndichidziwitso chaching'ono, mungagwiritse ntchito COUNT () ntchito mosiyanasiyana.