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How do you check for extraneous attributes in DBMS?
Formal definition of Extraneous Attribute For example, suppose F contains AB → CD, A → E, and E → C. To check if C is extraneous in AB → CD, we compute the attribute closure of AB under F’ = {AB → D, A → E, and E → C}. The closure is ABCDE, which includes CD, so we infer that C is extraneous.
What is extraneous attribute?
Extraneous attributes: An attribute of a functional dependency is said to be extraneous if we can remove it without changing the closure of the set of functional dependencies.
Which of the following can be a multivalued attribute?
8. Which of the following can be a multivalued attribute? Explanation: Name and Date_of_birth cannot hold more than 1 value.
What do you mean by canonical cover?
A canonical cover for F (a set of functional dependencies on a relation scheme) is a set of dependencies such that F logically implies all dependencies in , and. logically implies all dependencies in F. The set has two important properties: No functional dependency in contains an extraneous attribute.
What do you mean by multivalued dependency?
Multivalued Dependency(MD) • Multivalued dependency occurs when two attributes in a table are independent of each. other but, both depend on a third attribute. • A multivalued dependency consists of at least two attributes that are dependent on a. third attribute that’s why it always requires at least three attributes.
How to find if an attribute is extraneous?
Case 1 (LHS): To find if an attribute A in α is extraneous or not. That is, to test if an attribute of Left Hand Side of a functional dependency is Extraneous or not. Step 2: If ( {α} – A)+ contains all the attributes of β, then A is extraneous. Case 2 (RHS): To find if an attribute A in β is extraneous or not.
How to check if a dependency is extraneous in Alpha?
In chapter 8.4.3 in a dependency alpha->beta , If A ∈ aplha to check if A is extraneous, let (gamma = alpha − { A}), and check if (gamma → beta) can be inferred from F . To do so, compute gamma+ (the closure of gamma ) under F if gamma+ includes all attributes in beta, then A is extraneous in alpha
Why is B an extraneous attribute in the LHS?
Because, A can determine B (A → BC), B can determine C (B → C). Hence, A can determine C also (Transitivity rule). Hence, it is inappropriate to repeat or check an attribute many times. B is extraneous in the LHS of AB → D.