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Questions tagged [functional-dependencies]

A functional dependency is a constraint between two sets of attributes in a relation, in relational algebra, databases and type systems.

Functional dependencies (FD) are fundamental to the process of normalization.

Given a relation R, a set of attributes X in R is said to functionally determine a set of attributes Y, also in R, (written X → Y) if, and only if, whenever two tuples coincide on all attributes of X, they also coincide on all attributes of Y. Sound, complete and non-redundant axiomatisation of functional dependencies is given by Armstrong rules. Functional dependencies are also used in the Haskell programming language to describe relations between types, to support type level relational programming.

In other words, a dependency FD: X → Y means that the values of Y are determined by the values of X. Two tuples sharing the same values of X will necessarily have the same values of Y.

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Can a table be in 3NF with no primary keys?

1. A table is automatically in 3NF if one of the following holds: (i) If a relation consists of two attributes. (ii) If 2NF table consists of only one non key attribute. 2. If X → A is a dependency, then the table is in 3NF, if one of the…
Zephyr
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What is the difference between primary key, candidate key and super key in terms of functional dependency

What is the difference between primary key, candidate key and superkey in terms of functional dependency? I know the difference, but I don't know their difference in terms of functional dependencies.
Heng Zhao
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How can I find candidate keys?

Example: Let R = (A, B, C, D) Let F = {C -> AD, AB -> C} Then how can I find the candidate keys? The answer is {AB, BC} Why?
Na Jun Yeop
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Partial dependency

Suppose we have a relation R(A,B,C,D,E) with candidate keys {A,B} , {C,D} and let E be a non prime attribute. If there is a dependency C->E then it is partial dependency as E depends on a part of candidate key {C,D} Is dependency BC->E partial?
Zephyr
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Relational algebra - Functional Dependencies

I just want to be sure... If i've two dependencies like a -> c b -> c Will it be the same as this one a b -> c
Zialt
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Functional dependency and many-to-many relationships

I have these fields: A book_id B book_title C book_isbn D book_year G reader_id H reader_name I reader_birthday L reader_phone M reader_email N reader_registration_date O loan_id P loan_date_issued S loan_date_for_return T …
Umanet Alex
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Normalizing the relation to third normal form

Given the follwing functional dependencies, it is a little bit confusing for me because third normal form says no non-prime attribute of R is transitively dependent on the primary key. So i removed the functional dependency C --> DE from table and…
Khan Saab
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Determining Functional Dependencies and Candidate Keys

I'm having trouble grasping the concept of functional dependencies. If for example we have a table: A B C D |1| 1| 10| 10| |1| 2| 0| 10| |1| 3| 0| 10| |1| 4| 0| 10| |1| 5| 10| 20| |1| 6| 0| 20| |1| 7| 0| 20| |1| 8| 0| 20| |1| 9| 0|…
Sirpeter89
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What are the candidate keys in the below relation

R(A,B,C,D,E) Functional Dependencies = {AC->E,B->D,E->A} I know ABC is a candidate key.Can BC be a candidate key too?
theavatar
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Confusion over BCNF

Schema: R(A,B,C,D,E,F,G,H,I,J) and functional dependencies FD = { A->DE, IJ->H, I->A, J->FG, G->BC } Question: Is relation in BCNF? Answer: It's not because A is not superkey. I'm aware under which conditions relation is in BCNF, but what confuses…
sss
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Functional dependency vs one-to-many (or many-to-many ) relationships

I understand what 1) a FD (functional dependency) is A --> B i.e for a value a1 in A there will be a corresponding unique value b1 in B. 2) and in a one-to-many relationship, for a value a1 in A there will be one or more corresponding values…
samshers
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Candidate Keys on Functional Dependencies?

The relation R=(A,B,C,D,E) and functional dependencies F are given as follows: F={A->BC, CD->E, B->D, E->A} E, BC and CD can be a candidate keys, but B cannot. Anyone could point me how this fact is calculated? I google it but couldn't understand…
user6639553
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Functional dependency F = {AB->CD , A->B} or A->C

I have R(A,B,C,D) and functional dependencies AB->CD and A->B I want to know or A->C is correct? I try found F closure by amstrong axioms: AB->C (decomposition from AB->CD) AB->D (decomposition from AB->CD) AB->AB (reflexivity) AB->A (decomposition…
Edgaras Karka
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FD and databases

I'm trying to find all the functional dependencies that hold in this table: The way I understand it is: If faculty determines office_hours then since Sun has 4 appearances in faculty, each one has to map to the same value for office_hours. But…
henryzo
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Database Management - closure of functional dependencies

What is the closure of these functional dependencies of a relation? A -> DC D -> B Ans: A -> BC (using pseudo transitivity rule). Am I correct or am I missing something?
user5228835
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