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How do you know if a functional dependency holds?
Functional dependency – For a given Left Hand Side (LHS) value of an attribute (or set of attributes) of a functional dependency, there should be at most one Right Hand Side (RHS) value of an attribute (or set of attributes). Then we would say that the functional dependency holds on that relation.
What is a functional dependency of a relation instance?
A functional dependency is a one-way relationship between two attributes, such that at any given time, for each unique value of attribute A, only one value of attribute B is associated with it throughout the relation. For example, assume that A is the customer number from the orders relation.
What does it mean for a functional dependency to hold?
Functional dependency is a relationship that exists when one attribute uniquely determines another attribute. If R is a relation with attributes X and Y, a functional dependency between the attributes is represented as X->Y, which specifies Y is functionally dependent on X.
How to check whether a functional dependency hold in a relation R?
Functional dependency – For a given Left Hand Side (LHS) value of an attribute (or set of attributes) of a functional dependency, there should be at most one Right Hand Side (RHS) value of an attribute (or set of attributes). Then we would say that the functional dependency holds on that relation. REG_NO → STUDENT_NAME.
What is a functional dependency in a table?
A functional dependency (FD) is a relationship between two attributes, typically between the PK and other non-key attributes within a table. For any relation R, attribute Y is functionally dependent on attribute X (usually the PK), if for every valid instance of X, that value of X uniquely determines the value of Y.
Which is not a candidate key in functional dependency?
Answer: (AE)+ = {ABECD} which is not set of all attributes. So AE is not a candidate key. Hence option A and B are wrong. (BCH)+ = {BCHDA} which is not set of all attributes. So BCH is not a candidate key. Hence option C is wrong. This article is contributed by Sonal Tuteja.
How to determine the dependencies of a key?
As a shortcut, determine your candidate keys and the attributes that depend on them. Your examples are correct because a username, regDate, type, and subscription all depend on the value of user_id. If username is unique and not null, it is a candidate key and also identifies the set of attributes.