# Database MCQ - ER-model & Relational Model

61:

Why do we have to create a primary index on a primary key

 A. Ease of searching B. Sequential ordering of a file C. Both (a) and (b) D. None of these Answer Report Discuss Option: C Explanation : Click on Discuss to view users comments. Write your comments here:
62:

In context with relational Algebra. Which of the following are unary operatiors.
1.Select
2. Project
3. Union
4. Product

 A. 1 and 3 only B. 2 and 4 only C. 1 and 2 only D. All are binary Answer Report Discuss Option: C Explanation : Click on Discuss to view users comments. Write your comments here:
63:

Let R is a relation schema, R(A, B, C, D) and F = {A —> B, B --> C, C —> A) is the set of functional dependency. Determine the key of relation ?

 A. A B. B C. C D. D Answer Report Discuss Option: D Explanation : Rule to find out the key in relation :- Key ---> Rest of Attributes As Key determies rest of the attributes  Key must include all attributes but can not be on RHS of any functional dependency i.e Only fields on RHS are determined.  An undetermined field must be in the key.  REST must include all attributes but  NOT on LHS  They don't determine anything  so they can not be part of key. If field is not in ANY FD, it must be part of the key Now considering these 5 points we get D as Key. Click on Discuss to view users comments. Jyoti said: (10:51am on Tuesday 22nd October 2013) A superkey is a set of attributes whose closure contains all the attributes. A candidate key = a minimal superkey and primary key is selected candidate key here A , B , C all closure have all arttibutes(ABC) only D is left.hence we have AD,BD,CD as candidate keys and anyone of them can be a primary key Write your comments here:
64:

Which of the following statement is correct?

 A. F |= x ---> x  iff y ⊆ x+ B. F |= y ---->x iff y ⊆ x+ C. F |=x  --->y iff  x+ ⊆ y D. F |= y --->x iff x+⊆ y Answer Report Discuss Option: A Explanation : Click on Discuss to view users comments. Write your comments here:
65:

Given the basic ER and relational model which of the following is incorrect?

 A. An entity can have more than one attrbute B. An attribute of an entity can be composite. C. In row of a relational table, an attribute have more than one value. D. In row of a relational table, an attribute can have exactly one vlaue or a  NULL value. Answer Report Discuss Option: C Explanation : Click on Discuss to view users comments. Write your comments here: