# Thread: 5th Normal Form definition - I have trouble understanding it

1. Registered User
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Jun 2013
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## 5th Normal Form definition - I have trouble understanding it

Hi everyone,
I have a trouble understanding one definition of the 5th NF. My professor gave me a chapter about it from a book (I don't know which one is it, it's very old), and the definition of PJNF is as follows:

1. R is in PJNF with respect to F if for every JD *[R1, R2,...Rn] implied by F, every Ri is a superkey for R.

2. Revised definition is: R is in PJNF with respect to F for every JD *[r1,r2..rn] implied by F that applies to R, *[R1, R2, ..., Rn] is implied by the key FDs of R.

if R = A B C and F= { A->BC, C-> AB, *[AB, BC] }

I understand the first, every JD has a superkey in it, but I'm really having trouble grasping the second definition. Could anyone help me and explain why R doesn't meet the second criterion.
Thanks.

2. Resident Curmudgeon
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Feb 2004
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My first suggestion is to read (and maybe print) the Rettig Normalization Poster. It is the best introduction I've ever seen to understanding the first five normal forms for us mere mortals.

If that doesn't clear things up, there is a more technical discussion of Database Normalization that I find helpful at Wikipedia.

Obviously the text approved by your instructor is the authority for your class, but these resources may help you understand more of the concept so that the text that was provided will make more sense to you.

-PatP

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