Understanding 3NF: plain English please - database

I am working through an example problem in which we are trying to identify which of the following relations is in Third Normal Form (3NF). Here are the relations we are given:
R1(ABCD)
ACD -> B   AC -> D   D -> C   AC -> B
R2(ABCD)
AB -> C   ABD -> C   ABC -> D   AC -> D
R3(ABCD)
C -> B   A -> B   CD -> A   BCD -> A
R4(ABCD)
C -> B   B -> A   AC -> D   AC -> B
I know the answer is R1 is in 3NF, but I'm having a hard time understanding the steps to go about determining what violates 3NF. Can someone break it down in plain English for each of the relations? It would be extremely helpful if you can show me in steps how each relation might violate one of the 3NF rules:
X -> A, then A is a subset of X
X is a superkey
A is a part of some key for R
For R1, the first step I take is breaking it down into closures:
ACD+ = ABCD
AC+ = ABCD
D+ = C
ACD and AC are superkeys, which satisfy rule 2.
1. D -> C, but C is not a subset of D. Rule 1 violated.
2. D is not a superkey. Rule 2 is violated.
3. C is a part of some key for R. C is a part of AC and ACD. So, rule 3 is upheld?
Not sure if I'm even doing these steps right, so please break it down as simple as possible for someone struggling with these concepts. Thanks.

The best definition I've found for a relation that is in third normal form (3NF) is the following:
A relation schema R is in 3NF if, whenever a function dependency X -> A holds in R, either
(a) X is a superkey of R, or
(b) A is a prime attribute of R.
Now there are three definitions that need clarification, key,superkey, and prime attribute.
For the definitions we will use examples from the R1 relation to describe them:
R1(ABCD)
ACD -> B   AC -> D   D -> C   AC -> B
key: A key is the attribute that determines every attribute of the relation. In other words, it is the set of attributes that will give you all the other attributes of the relation that are not in the set. In relation R1 of the above example, the keys are AC and AD. Why is AC a key? Because by knowing attributes A and C you can determine the remaining attributes, B and D. Why is AD a key? The same reason. A and D will ultimately determine B and C.
superkey: A superkey is basically a superset of a key. A superkey will contain the key always and potentially more attributes. In the previous example, AC is a key. Thus AC, ACD, ACB, etc. are superkeys. Note that a key itself is a superkey.
prime attribute: A prime attribute is basically an attribute that is part of a key. Thus A and C are prime attributes as they are part of the key AC. Take note however, the difference between a key and superkey. For the super key ACB, B is not a prime attribute since B is not part of the key. Just think of a prime attribute as a subset of a key.
Now let's look at the four relations:
R1(ABCD)
ACD -> B   AC -> D   D -> C   AC -> B
R2(ABCD)
AB -> C   ABD -> C   ABC -> D   AC -> D
R3(ABCD)
C -> B   A -> B   CD -> A   BCD -> A
R4(ABCD)
C -> B   B -> A   AC -> D   AC -> B
For each relation we will write down the keys and the prime attributes. Then we will see if the definition is satisfied.
R1:
keys: AC, AD
prime attributes: A, C, D
ACD -> B: Left side is a superkey. Satisfies (a).
AC -> D: Left side is a key and thus a superkey. Satisfies (a).
D -> C: Left side is not a superkey. Does not satisfy (a). However, right side is a prime attribute. Satisfies (b).
AC -> B: Left side is a key. Satisfies (a).
Either (a) or (b) is satisfied in all cases. Thus R1 is in 3NF.
R2:
keys: AB
prime attributes: A, B
AB -> C: Left side is a key and thus a superkey. Satisfies (a).  
ABD -> C: Left side is a superkey. Satisfies (a).
ABC -> D: Left side is a superkey. Satisfies (a).
AC -> D: Left side is not a superkey. Does not satisfy (a). Right side is not a prime attribute. Does not satisfy (b).
Since (a) or (b) is not satisfied in all cases, R2 is not in 3NF.
R3:
keys: CD,
prime attributes: C, D
C -> B: Left side is not a superkey. Does not satisfy (a). Right side is not a prime attribute. Does not satisfy (b).
Since we have already found a case that does not satisfy either (a) or (b), we can immediately conclude that R3 is not in 3NF.
R4:
keys: C
prime attributes: C
C -> B: Left side is a key and thus a superkey. Satisfies (a).  
B -> A: Left side is not a superkey. Does not satisfy (a). Right side is not a prime attribute. Does not satisfy (b).
Again, we can stop here as the second case satisfies neither (a) nor (b). The relation R4 is not in 3NF.

A simplified expression of 3NF is "A relation is in 3NF if every attribute transitively dependent on a key is a key attribute."1 A key attribute is an attribute that's part of any candidate key.
R3 is one of the simpler ones to analyze with respect to 3NF.
R3(ABCD)
C -> B
A -> B
CD -> A
BCD -> A
The only candidate key is CD.
Is there a transitive dependency? Yes, there is: CD->A, and A->B. B is transitively dependent on the key.
Is B a key attribute? No, it's not; CD is the only key.
So R3 is not in 3NF.
R4 is similar. C is the only candidate key.
Is there a transitive dependency? Yes, there is: C->B, and B->A.
Is A a key attribute? No, it's not; C is the only key.
So R4 is not in 3NF.
In R1, the candidate keys are AC and AD.
Is there a transitive dependency? Yes, there is: AC->D, and D->C.
Is C a key attribute? Yes, it is.
So R1 is in 3NF.
"A New Normal Form for the Design of Relational Schemata", Carlo Zaniolo, in ACM Transactions on Database Systems, Vol. 7, No. 3, September 1982, p 492. More recent work uses prime attribute to refer to attributes that are part of any candidate key and non-prime attribute to refer to attributes that are not part of any candidate key.

Let me explain in simple words:
For the given relation,
R1(ABCD),the functional dependencies are:
ACD -> B
AC -> D
D -> C
AC -> B
Condition to be in 3NF
X->Y here X is a Super key when Y is non-prime attribute else it can be any attribute
Prime attributes are the attributes which belongs to the super key
Non-prime attributes are the attributes which do not belongs to the super key
Let us come back to relation R1,
AC+=ABCD
AD+=ABCD
ACD+=ABCD and
D+=DC
Thus,we get AD,AC,ACD as our Super keys
And A,C,D are prime attributes and B is a non-prime attributes
ACD->B
This Functional dependency is in 3NF as ACD is a super key and B is prime attribute
AC->D
This Functional dependency is also in 3NF as AC is a super key and D is prime attribute
D->C
This Functional dependency is also in 3NF as D is a prime attribute and C is also prime attribute
AC->B
This Functional dependency is also in 3NF as AC is a super key and B is non-prime attribute
Thus,the relation is not in 3NF only when non-prime attributes
does not depend on super key
Hope,this helps!

In simple english, here are the 3 Normal Forms:
1NF: The existence of "the key" ensures that the table is in 1NF(Key must be there).
2NF:It is required that "each" non-key attributes be dependent on "the whole key" to ensure 2NF.
3NF: further requiring that "each" non-key attributes be dependent on "nothing but the key" ensures 3NF.
Now, for this:
R1(ABCD) ACD -> B   AC -> D   D -> C   AC -> B
Look at these ACD -> B and AC -> B: clearly violates the 2NF condition. Forget 3NF, this relation is not even in 2NF. "the whole key"-->concept does not hold.
I think, you have proved the same using SET.

Related

Relations in 3NF format

I was doing some textbook questions which contain this following one.
Which of the following relations is in Third normal form (3NF)?
a) R(ABCD) FD's: ABD → C ; CD → A ; AC → B ; AC → D
b) R(ABCD) FD's: ABD → C ; A → B ; AB → C ; B → A
c) R(ABCD) FD's: AB → C ; ABD → C ; ABC → D ; AC → D
d) R(ABCD) FD's: AD → C ; D → A ; A → C ; ABC → D
I feel that all four choices are wrong but I do not have the answer to it. Could anybody help me with it? R represents relationship, FD is short term for functional dependency.
(Disclaimer: this is not from test/homework which gives any credit, I just want to make sure that I understand the concept correctly)
The way to prove something is in a certain NF form is by trying to break it. It is hard to describe why something is rather than is not in 3NF. That being said, could you tell me why you think each one does not satisfy 3NF so we can work from there?
This is what I've figured out:
b) Breaks 2NF since candidate key ABD can be broken down into AB which determines non-prime C.
c) Only candidate key is AB. AB -> C. D is determined through a transitive relationship ABC -> D. Breaks 3NF.
d) Candidate key BA determines C. But A -> C, therefore this breaks 2NF.
I could not find anything wrong with a, so that would be my guess to the right answer.
Let me explain it in simple words:
Condition for 3NF:
The non-prime attributes should be dependent only on super key
For option A,
By closure property,
AC+=ABCD
Thus A,C are prime attributes and B,D are non-prime attributes
Since all non-prime attributes are dependent on super key AC
The option A is in 3NF
For option B,
By closure property,
ABD+=ABCD
Thus,A,B,D are prime attributes and C is non-prime attributes
Since C, a non-prime attribute depends on AC which is not a super key,
option B is not in 3NF
For option C,
By closure property,
AB+=ABCD
Thus,A,B are prime attributes and C,D are non-prime attributes
Since D a non-prime attribute depends on AC which is not a super key
option C is not in 3NF
For option D,
By closure property,
AB+=ABCD
Thus,A,B are prime attributes and C,D are non-prime attribute
Since C, a non-prime attribute depends on AD which is not a super key,
option D is not in 3NF
Hope,it helps!!

Concept Check -- Key Determination from Functional Dependencies

This is more of a concept check than anything, but suppose I have a relation R on attributes ABCD with the functional dependencies B -> ACD and C -> D. The solitary key for this relation is B, and a superkey for this relation is BC, correct?
The only candidate key in your example is indeed B. Any collection of attributes containing B will be a superkey, e.g., B itself, AB, BC or ABD.
candidate key is B.
As all the attributes can be derived from B, B becomes superkey..

Convert To BCNF

I'm given the following relation:
R(A,B,C,D,E)
and the following Functional Dependencies:
F1 - AC -> D
F2 - D -> E
F3 - E -> A
I am attempting to convert this to BCNF form.
The first step I took was to figure out the possible keys for the relation.
Keys: ABC, BCD, BCE
Then I checked to see if the functional dependencies fit BCNF form, they do not.
So I attempted to decompose and got the following:
R1(A,C,D) AC->D Keys: AC
R2(D, E) D->E Keys: D
I believe at this point that the relations are in BCNF form and are lossless, but are not dependency preserving.
Does this seem like the right technique for this sort of thing? Is there a step or two I might have missed?
I believe there is an error in your BCNF. Between R1 and R2, you've lost B.
Keys, ABC, BCD and BCE are correct. (You require B, C and one of either A, D or E).
R1 (A, B, C, D), key ABC and R2 (D, E) with key D is a valid normalization.

Removing Anomalies from tables

I have this table and I am supposed to re-design it to remove all anomalies from it. I will not give the table but the dependencies.
The functional dependencies are
(Product, Store) -> Dept
Store -> Manager
Dept-> Assist
(Product, Store)->Price
Product-> Weight
Product-> Manufact
Manufact-> Manuloc
Product and Store are the keys in this relation
Ok so I have to remove the anomalies by breaking up the table and creating new ones and state what will be in each table. I am unsure how to do that.
Apply Armstrong's axioms and the rules derived from them. (I'm sure this is in your textbook. Check the index.) For example, given these two functional dependencies (FDs):
(Product, Store) -> Dept
Dept -> Assist
you can apply the transitivity rule to determine an unstated FD:
(Product, Store) -> Assist
From the FDs you're given, derive an irreducible set of FDs; this set determines your tables.
This example is from Date's Introduction to Database Systems. (He gives a more complete treatment of it.) Given
A -> BC
B -> C
A -> B
AB -> C
AC -> D
rewrite for right-hand singletons using Armstrong's axioms:
A -> B
A -> C
B -> C
A -> B
AB -> C
AC -> D
The FD A -> B occurs twice; we can discard one without losing information. We can reduce AC -> D to A -> D, and we can eliminate AB -> C. And we can eliminate A -> C, leaving
A -> B
B -> C
A -> D
From this irreducible set of FDs, you'd derive the two tables
A -> BD
B -> C

BCNF Decomposition

I am trying to figure out the correct steps in performing a BCNF decomposition. I found this example, but I do not understand how to perform the correct steps.
Schema = (A,B,C,D,E,F,G,H)
FD's + {A -> CGH, AD->C, DE->F, G->G}
Could someone show the correct steps?
Determine a minimal cover using your FD's:
{A -> C, A -> G, A -> H,
B -> nothing,
C -> nothing,
D -> nothing,
E -> nothing,
F -> nothing
G -> nothing
H -> nothing
DE -> F}
Note AD -> C drops out because A alone determines C which implies D is redundant in the FD (see Armstrong's Axioms - Augmentation).
3NF and BCNF definitions relate to dependencies about compund keys. The only compound key
you have here is DE. Neither D or E participate in any other non-null FD's
so eliminating transitive dependencies and ensuring that dependent attributes rely on the
'key, the whole key, and nothing but the key' is not an issue here.
Break into relations so that the FD left hand side is the key and the right hand sides
are the non-key dependent attributes of that key:
[Key(A), C, G, H]
[Key(D, E), F]
Now eliminate these attributes from the cover, whatever is left are standalone relations.
[Key(B)]
This should be in 3NF/BCNF

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