Cad é an Ghnáthchineál Boyce-Codd (BCNF)?

by Mike Chapple

Laghdaíonn BCN iomarcaíochtaí agus méadaíonn ionracas sonraí

Is é an sprioc atá ag Gnáthchineál Boyce-Codd (BCNF) ná ionracas sonraí a mhéadú trí cholúin agus táblaí bunachar sonraí gaol a eagrú chun normalú bunachar sonraí a bhaint amach. Tarlaíonn normalú bunachar sonraí nuair a bhíonn caidrimh bunaithe idir táblaí agus nuair a bhíonn rialacha sainmhínithe ag na táblaí chun an bunachar sonraí a dhéanamh níos solúbtha agus na sonraí a chaomhnú.

Is iad na spriocanna a bhaineann le normalú bunachar sonraí ná sonraí iomarcacha a dhíchur agus chun a chinntiú go mbíonn ciall ar chleithiúlachtaí sonraí.

Déantar bunachar sonraí a normalú nuair nach stóráiltear na sonraí céanna i níos mó ná aon tábla amháin agus nuair nach stóráiltear ach sonraí gaolmhara i dtábla.

Bunús Fhoirm Ghnáth Boyce-Codd

Tar éis sraith treoirlínte a chinntiú go ndéantar bunachar sonraí a normalú. Déantar tagairt do na treoirlínte seo mar fhoirmeacha gnáth agus déantar iad a uimhriú ó cheann trí chúig. Déantar cur síos ar bhunachar sonraí coibhneasta mar a dhéantar gnáthú má chomhlíonann sé na chéad trí fhoirm: 1NF, 2NF, agus 3NF.

Cruthaíodh BCNF mar shíneadh ar an tríú foirm gnáth, nó 3NF, i 1974 ag Raymond Boyce agus Edgar Codd. Bhí na fir ag obair chun scéimeanna bunachar sonraí a chruthú a iomarcaíonn iomarcaíochtaí leis an sprioc chun am ríomhaireacht a laghdú. Cuireann an tríú gnáth-fhoirm colúin nach bhfuil ag brath ar an eochair phríomhúil chomh maith leis na treoirlínte a chomhlíonadh sa chéad agus an dara foirmeacha gnáth. Bíonn BCNF, a dtugtar uaireanta mar 3.5NF, ag freastal ar riachtanais uile 3NF agus éilíonn sé nach bhfuil aon spleáchas ag eochracha iarrthóra ar thréithe eile i dtábla.

Ag an am a chruthaíodh BCNF, bhí Boyce ar cheann de na príomhfhorbróirí ar Theanga Iarratas Struchtúrtha Béarla (SQL), rud a d'éirigh le sonraí a aisghabháil trí úsáid a bhaint as samhail coibhneasta Codd. Sa tsamhail seo, d'áitigh Codd gur féidir laghdú a dhéanamh ar chastacht struchtúrach na mbunachar sonraí, rud a chiallaíonn go bhféadfadh ceisteanna a bheith níos cumhachtaí agus níos solúbtha.

Ag baint úsáide as a léargas bunachar sonraí gaolmhar, sainmhínigh Codd treoirlínte 1NF, 2NF, agus 3NF. D'fhoireann sé le Boyce chun BCNF a shainiú.

Eochracha Iarrthóirí agus BCNF

Is é an t-iarrthóir ná colún nó teaglaim de cholúin i dtábla a chruthaíonn eochair uathúil sa bhunachar sonraí. Is féidir an meascán de na tréithe a úsáid chun taifead bunachar sonraí a aithint gan tagairt a dhéanamh d'aon sonraí eile. Is féidir go mbeadh eochracha iarrthóra i ngach tábla, agus is féidir le ceann ar bith díobh cáiliú mar phríomh-eochair. Níl aon eochair bhunscoile amháin ann i dtábla.

Ní mór do na hiarrthóirí a bheith uathúil.

Tá gaol i BCNF má tá gach cinntitheoir ina eochair iarrthóra. Smaoinigh ar an mbord bunachar sonraí a stóráiltear faisnéis an fhostaí agus go bhfuil na tréithe , , , agus . </p> <p> Sa tábla seo, socraíonn an réimse <Employee_Id> first_name agus last_name. Ar an gcaoi chéanna, socraíonn an tuple (<First_name>, <Last_name>) <Employee_id>. </p> <table><tbody><tr><td> <strong>Id Fhostaithe</strong> </td><td> <strong>Ainm</strong> </td><td> <strong>Ainm dheireanach</strong> </td><td> <strong>Teideal</strong> </td></tr><tr><td> 13133 </td><td> Emily </td><td> Smith </td><td> Bainisteoir </td></tr><tr><td> 13134 </td><td> Jim </td><td> Smith </td><td> Comhlach </td></tr><tr><td> 13135 </td><td> Emily </td><td> Jones </td><td> Comhlach </td></tr></tbody></table><p><br> Is é an t-eochair iarrthóra don <a href="https://ga.eyewated.com/gluais-tearmai-coiteanna-bunachar-sonrai/">bhunachar sonraí</a> seo an <Employee_Id> toisc go bhfuil sé an t-aon luach nach féidir le sraith eile a úsáid. </p> </div> <div class="amp-related-wrapper"> <h2>Alike posts</h2> <div class="amp-related-content"> <a href="https://ga.eyewated.com/cruthaigh-tabla-le-sql-server-2012/"> <amp-img src="https://exse.eyewated.com/pict/c64fed84b4c932c4-120x86.jpg" width="120" 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