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Subject: Re: BNF og 3NF


Author:
Hjįlmtżr
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Date Posted: 22:10:35 07/Dec/2007
In reply to: Sigurjón 's message, "BNF og 3NF" on 14:21:15 07/Dec/2007
 >Er žaš ekki rétt skiliš hjį mér aš ef aš tafla er į
>3NF og hefur ósamsettann frumlykil žį er hśn lķka į
>BNF?

Ja, žetta er ekki alveg rétt.

Eini munurinn į skilgreiningunum į 3NF og BCNF (į bls.616-617) er skilyršiš "A er hluti af lykli fyrir R" sem mį vera ķ 3NF, en ekki ķ BCNF. Ef ALLIR lyklar fyrir R eru ósamsettir žį getur A ekki veriš hluti af lykli og žetta skilyrši kemur ekki upp og fyrir žį töflu er 3NF jafngilt BCNF. En ef einhver lykill er samsettur žį getur skilyršiš komiš inn og greint į milli 3NF og BCNF.

Tökum sem dęmi venslin R(A,B,C,D) meš fallįkvešurnar A->BCD, BC->A og D->C. Lyklarnir hér eru A, BC og BD (muniš aš lyklar eru "minimal" mengi dįlka sem įkvaršar alla ašra dįlka). Hér er lykillinn A ósamsettur, en BC og BD samsettir. Fallįkvešan D->C veldur žvķ aš venslin eru į 3NF, en ekki į BCNF (D er ekki yfirlykill).
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Re: BNF og 3NFNo name23:48:40 07/Dec/2007

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