ER and Relational Models

Question 51

In a relational database model, NULL values can be used for all but which one of the following ?

• To allow duplicate tuples in the table by filling the primary key column(s) with NULL.
• To avoid confusion with actual legitimate data values like 0 (zero) for integer columns and ’’ (the empty string) for string columns.
• To leave columns in a tuple marked as ’’unknown’’ when the actual value is unknown.
• To fill a column in a tuple when that column does not really ”exist” for that particular tuple.

Question 52

A relation R = {A, B, C, D, E, F,G} is given with following set of functional dependencies: F = {AD → E, BE → F, B → C, AF → G} Which of the following is a candidate key ?

• A
• AB
• ABC
• ABD

Question 53

Drop Table cannot be used to drop a Table referenced by __________ constraint. (a)Primary key (b)Sub key (c)Super key (d)Foreign key

• (a)
• (a), (b) and (c)
• (d)
• (a) and (d)

Question 54

For a weak entity set to be meaningful, it must be associated with another entity set in combination with some of their attribute values, is called as:

• Neighbour Set
• Strong Entity Set
• Owner Entity Set
• Weak Set

Question 55

The \'command\' used to change contents of one database using the contents of another database by linking them on a common key field?

• Replace
• Join
• Change
• Update

Question 56

The join operation can be defined as

• a cartesian product of two relations followed by a selection
• a cartesian product of two relations
• a union of two relations followed by cartesian product of the two relations
• a union of two relations

Question 57

Which commands are used to control access over objects in relational database?

• CASCADE & MVD
• GRANT & REVOKE
• QUE & QUIST
• None of these

Question 58

What kind of mechanism is to be taken into account for converting a weak entity set into strong entity set in entity-relationship diagram?

• Generalization
• Aggregation
• Specialization
• Adding suitable attributes

Question 59

Identify the minimal key for relational scheme R(A, B, C, D, E) with functional dependencies F = {A → B, B → C, AC → D}

• A
• AE
• BE
• CE

Question 60

Immunity of the external schemas (or application programs) to changes in the conceptual schema is referred to as:

• Physical Data Independence
• Logical Data Independence
• Both (a) and (b)
• None of the above