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51.	What is the method of specifying a primary key in a schema description?(a) By writing it in bold letters(b) By underlining it using a dashed line(c) By writing it in capital letters(d) By underlining it using a bold lineThe question was posed to me at a job interview.This intriguing question comes from Introduction to the Relational Model in chapter Introduction to Relational Model and Sql of RDBMS
Answer» The correct option is (d) By UNDERLINING it using a bold line The best EXPLANATION: We can SPECIFY a primary key in SCHEMA description by underlining the respective attribute with a bold line.

52.	An attribute is a __________ in a relation.(a) Row(b) Column(c) Value(d) TupleThe question was asked in semester exam.The above asked question is from Introduction to the Relational Model topic in division Introduction to Relational Model and Sql of RDBMS
Answer» The correct option is (B) Column For EXPLANATION: An ATTRIBUTE is a column in a RELATION. A tuple is a row in a relation.

53.	What does the “x” operator do in relational algebra?(a) Output specified attributes from all rows of the input relation. Remove duplicate tuples from the output(b) Output pairs of rows from the two input relations that have the same value on all attributes that have the same name(c) Output all pairs of rows from the two input relations (regardless of whether or not they have the same values on common attributes)(d) Returns the rows of the input relation that satisfy the predicateThis question was posed to me in an international level competition.This key question is from Introduction to the Relational Model topic in chapter Introduction to Relational Model and Sql of RDBMS
Answer» Correct CHOICE is (c) Output all pairs of rows from the two input relations (REGARDLESS of whether or not they have the same values on common attributes) Easy explanation: The “x” operator outputs all pairs of rows from the two input relations (regardless of whether or not they have the same values on common attributes). This OPERATION is CALLED as the Cartesian product operation and is similar to the Cartesian product of sets.

54.	What action does⋈operator perform in relational algebra(a) Output specified attributes from all rows of the input relation and remove duplicate tuples from the output(b) Outputs pairs of rows from the two input relations that have the same value on all attributes that have the same name(c) Output all pairs of rows from the two input relations (regardless of whether or not they have the same values on common attributes)(d) Return rows of the input relation that satisfy the predicateI had been asked this question in class test.My question is from Introduction to the Relational Model in chapter Introduction to Relational Model and Sql of RDBMS
Answer» Correct answer is (a) OUTPUT specified attributes from all rows of the input relation and remove DUPLICATE tuples from the output The BEST EXPLANATION: ⋈ Outputs specified attributes from all rows of the input relation. Remove duplicate tuples from the output. The operation is called the join operation.

55.	What is a foreign key?(a) A foreign key is a primary key of a relation which is an attribute in another relation(b) A foreign key is a superkey of a relation which is an attribute in more than one other relations(c) A foreign key is an attribute of a relation that is a primary key of another relation(d) A foreign key is the primary key of a relation that does not occur anywhere else in the schemaThis question was posed to me in a job interview.Origin of the question is Introduction to the Relational Model in portion Introduction to Relational Model and Sql of RDBMS
Answer» Correct ANSWER is (c) A FOREIGN key is an attribute of a relation that is a primary key of ANOTHER relation The explanation is: A foreign key is an attribute of a relation that is INITIALLY a primary key of another relation. A foreign key USAGE preserves referential integrity.

56.	What is an Instance of a Database?(a) The logical design of the database system(b) The entire set of attributes of the Database put together in a single relation(c) The state of the database system at any given point of time(d) The initial values inserted into the Database immediately after its creationThe question was asked by my college director while I was bunking the class.I'm obligated to ask this question of Introduction to the Relational Model in section Introduction to Relational Model and Sql of RDBMS
Answer» Correct answer is (c) The state of the DATABASE SYSTEM at any given point of TIME Explanation: The state of the database system at any given point of time is called as an Instance of the database.

57.	Choose the correct statement regarding superkeys(a) A superkey is an attribute or a group of multiple attributes that can uniquely identify a tuple(b) A superkey is a tuple or a set of multiple tuples that can uniquely identify an attribute(c) Every superkey is a candidate key(d) A superkey is an attribute or a set of attributes that distinguish the relation from other relationsThis question was posed to me during an interview.This intriguing question comes from Introduction to the Relational Model topic in portion Introduction to Relational Model and Sql of RDBMS
Answer» Correct OPTION is (a) A superkey is an attribute or a group of multiple attributes that can uniquely IDENTIFY a tuple The explanation: A superkey is an attribute or a set of multiple attributes that can uniquely identify a tuple. It is USED to differentiate between tuples.