Database miscellaneous


Database miscellaneous

  1. Which one of the following statements about normal forms is false?









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    The statement false is Lossless, dependencypreserving decomposition into BCNF is always possible.
    Lossless, dependency preserving decomposition is possible in certain cases only. It is important to note that BCNF is not dependency preserving in all cases but in some cases only.

    Correct Option: C

    The statement false is Lossless, dependencypreserving decomposition into BCNF is always possible.
    Lossless, dependency preserving decomposition is possible in certain cases only. It is important to note that BCNF is not dependency preserving in all cases but in some cases only.


  1. Consider a relational schema R = (A, B, C, D, E, H) on which the following functional dependencies hold: {A → B, BC → D, E → C, D → A}. What are the candidate keys of R?









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    A → B, BC → D, E → C, D → A
    We start form set of all the attributes and reduce them using given functional dependences
    ABCDEH ABCDEH
    ABCEH {BC- → D}
    ABEH{E → C} BCDEH {D → A}
    AEH {A → B} BEH {E → C}
    ABCDEH ACDEH {A → B} ADEH{E → C} DEH{D → A}
    So candidate keys are AEH, BEH & DEH .

    Correct Option: D

    A → B, BC → D, E → C, D → A
    We start form set of all the attributes and reduce them using given functional dependences
    ABCDEH ABCDEH
    ABCEH {BC- → D}
    ABEH{E → C} BCDEH {D → A}
    AEH {A → B} BEH {E → C}
    ABCDEH ACDEH {A → B} ADEH{E → C} DEH{D → A}
    So candidate keys are AEH, BEH & DEH .



  1. The following table has two attributes A and C where A is the primary key and C is the foreign key referencing a with on-delete cascade.

    The set of all tuples that must be additionally deleted to preserve referential integrity when the tuple (2, 4) is deleted is









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    On delete cascade says that deletion of a primary key value should delete its all foreign key references.
    So in (2,4) 2 is primary key so tuples (5,2) & (7,2) should be deleted,
    but in (5,2) 5 is also a key so (9,5) also deleted cascade.
    Hence (c) is correct option.

    Correct Option: C

    On delete cascade says that deletion of a primary key value should delete its all foreign key references.
    So in (2,4) 2 is primary key so tuples (5,2) & (7,2) should be deleted,
    but in (5,2) 5 is also a key so (9,5) also deleted cascade.
    Hence (c) is correct option.


  1. The following functional dependencies are given
    AB → CD, AF → D, DE → F, C → G, F → E, G → A
    Which one of the following options is false?









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    Consider a+
    result = a
    Also consider, a = a+
    While do
    for each functional dependency X → Z in f
    do begin
    if X C a (result), then
    result = result YZ
    end
    compute {AF}+ = {AF} = {AFD} AF -> D
    = {AFDE} F -> E

    Correct Option: C

    Consider a+
    result = a
    Also consider, a = a+
    While do
    for each functional dependency X → Z in f
    do begin
    if X C a (result), then
    result = result YZ
    end
    compute {AF}+ = {AF} = {AFD} AF -> D
    = {AFDE} F -> E



  1. Which one of the following statements is false?









  1. View Hint View Answer Discuss in Forum

    Statement (A) Any relation with two attributes is in BCNF is true as with the rules of BCNF, any relation with two attributes gets into Boyce codd normal form.
    Statement (B) A relation in which every key has only one attribute is 2NF is true as 2NF allows only the relation where a key has only one attribute.
    Statement (C) A prime attribute can be transitively dependent on a key in a 3NF relation is also true as prime attribute has to be transitively dependent on a key in 3NF.
    Statement (D) A prime attribute can be transitively dependent on a key in a BCNF relation is false as this is a feature of 3NF and not BCNF.

    Correct Option: D

    Statement (A) Any relation with two attributes is in BCNF is true as with the rules of BCNF, any relation with two attributes gets into Boyce codd normal form.
    Statement (B) A relation in which every key has only one attribute is 2NF is true as 2NF allows only the relation where a key has only one attribute.
    Statement (C) A prime attribute can be transitively dependent on a key in a 3NF relation is also true as prime attribute has to be transitively dependent on a key in 3NF.
    Statement (D) A prime attribute can be transitively dependent on a key in a BCNF relation is false as this is a feature of 3NF and not BCNF.