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Let R(A, B, C, D) be a relation schema and F = {A →...

  • Q. Let R(A, B, C, D) be a relation schema and F = {A  BC, AB → D, B → C} be the set of functional dependencies defined over R. Which of the following represents the closure of the attribute set {B}?
  • filter_dramaExplanation
    Answer is : B

    Concept:- 

    Functional Dependencies:- Functional dependencies are the result of the interrelationship between attributes of any relation. Suppose in relation R, X and Y are two the two subsets of the set of attributes, Y is said to be functionally dependent on X if a given value of X uniquely determines the value of Y.

    It is denoted by X → Y. It means Y depends upon X or X holds Y.

    Here X is known as a determinant of functional dependency.

    Explanation:-

    Here, given R( A, B, C, D ) be a relation schema

    and, F = {A → BC, AB → D, B → C} be the set of functional dependencies defined over R.

    Closure of attributes {B}:-

    B= { B }

         = { B , C } ( using B →C )

    We can not determine any other attributes B and C  contained in the result set.

    Thus, 

    B+ = { B , C }

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Similar Questions

  • 1. A transaction is made to wait until all ________ locks held on the item are released
  • filter_dramaExplanation
    Answer is : A
    A transaction is made to wait until all compatible locks held on the item are released. This ensures that no other transaction is concurrently accessing the same item.
  • 2.

    Consider the following statements:

    S1: In distance vector routing, each router maintains a routing table indexed by, and containing one entry for each router in the network.

    S2: Link state routing requires more memory and computation as compared to distance vector routing.

    Which of the given statements is/are true?

  • filter_dramaExplanation
    Answer is : C

    Answer: Option 3

    Explanation

    Both S1 and S2 are true.

    In distance vector routing, each router maintains a routing table indexed by and containing one entry for each router in the network. This entry has two parts: the preferred outgoing line to use for that destination and an estimate of the distance to that destination. The distance might be measured as the number of hops.

    Compared to distance vector routing, link state routing requires more memory and computation. For a network with n routers, each of which has k neighbours, the memory required to store the input data is proportional to kn, which is at least as large as a routing table listing all the destinations.

  • 3. Which of the following statements is FALSE?
  • filter_dramaExplanation
    Answer is : A

    Option 1) A database relation can have more than one primary key, is a false statement.

    Explanation:-

    Every relation has at least one candidate key, because at least the combination of all its attributes has the uniqueness property. In the case of base relations (relations of a base table), one candidate key is designated as the primary key and the remaining candidate keys are called alternate keys.

    Key Points

    •  A database relation can have both primary key and secondary key.
    • A foreign key is an attribute or attribute combination of one relation(table) whose values are required to match those of the primary key of some other relation(table).
    • Also, the foreign key and the primary key should be defined on the same underlying domain.
    • A database relation can have more than one foreign key.
    • A candidate key is an attribute that can uniquely identify a row in a table.
  • 4. In data communication, jitter refers to ___________
  • filter_dramaExplanation
    Answer is : D

    The effective of a data communications system depends on four fundamental characteristics:

    Delivery, Accuracy, Timeliness and Jitter.

    Delivery: The system must deliver data to the correct destination; Data must be received by the intended device or user

    Accuracy: The system must deliver the data accurately; Data that have been altered in transmission and left uncorrected are unusable

    Timeliness: The system must deliver data in a timely manner; Data delivered late are useless

    Jitter: Jitter refers to the variation in the packet arrival time; It is the uneven delay in the delivery of audio or video packets
  • 5. Data unit at Data Link Layer is called
  • filter_dramaExplanation
    Answer is : B

    The correct answer is Frame

    • The Data-link layer takes the packets from the Network Layer and encapsulates them into frames. If the frame size becomes too large, then the packet may be divided into small-sized frames to make efficient flow control and error control.
    • Frames are a data-link layer data unit that is transmitted between network points. It contains complete addressing, necessary protocol, and control information. Before transmission of the frame, the data link layer protocol encapsulates the frame with a trailer and header.​
    • Assume a single link from A to B.
    • Assume service provided by the physical layer is that it will deliver bits to B in the order in which they are sent.
    • So Following are the tasks the data link layer have to do:
      • Error checking - all circuits have errors occasionally.
      • Flow regulation - Line has a finite data rate, and machines have a finite rate at which they can process incoming. Regulate flow so that slow receivers are not swamped by fast senders. 
    • The packet is a generic term for a piece of data that a higher layer wants to send.
    • The data link layer sends frames - small fixed (or max) length pieces of data. Frame length specific to hardware. The frame is packet encoded for transmission on this link.
    • As a packet travels across the Internet from A to B, it may go along multiple types of links (phone, fiber, wireless), each with different frame sizes and formats. The packet may be encoded in a different way for each link it travels on.  

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