CS Free MCQ Bank

[ Option B ]

A Secondary Key is a field or a group of fields that is used to retrieve or search records in a table but does not uniquely identify a record. 
For example, in a student table, attributes like name or city can be used as secondary keys to retrieve multiple matching records.

• Foreign Key is used to establish relationships between tables.
• ERD is a diagram used for database design.

[ Option D ]

In a relational database, a Super Key is any set of attributes that uniquely identifies tuples, but it may contain extra attributes and hence is not always minimal.
A Candidate Key is a minimal super key, meaning no subset of it can be a super key. Among candidate keys, one is chosen as the Primary Key to uniquely identify tuples.
Since the Primary Key itself is a candidate key, it cannot be a proper subset of another candidate key.

[ Option D ]

A column constraint is applied directly to a single column while defining a table.

• NOT NULL ensures that a column cannot have NULL values.
• PRIMARY KEY can be defined at the column level.
• CHECK restricts the values that can be stored in a column based on a condition.

[ Option C ]

Referential integrity constraint ensures that a Foreign Key in one relation (R2) must always refer to an existing Primary Key in another relation (R1).
If we insert a tuple in R2 with a foreign key value that does not exist as a primary key in R1, it violates referential integrity.
If a tuple in R1 is deleted but there are still tuples in R2 that reference it, the relationship becomes invalid, causing a referential integrity violation.

[ Option A ]

A table can have only one primary key. The primary key uniquely identifies each record in a table.
It may consist of one attribute or a combination of multiple attributes (composite primary key).

[ Option D ]

In a Relational Database Management System (RDBMS), keys are used to uniquely identify records in a table or relation. A super key is an attribute or a combination of attributes that ensures the uniqueness of each record. Among these super keys, a candidate key is a minimal set of attributes that can still uniquely identify a record, meaning it does not contain any unnecessary attributes.

In other words, every candidate key is a subset of a super key, but not all super keys are candidate keys because some may include extra attributes that are not needed for uniqueness.

[ Option A ]

Referential Integrity means that you cannot enter a value in a foreign key column unless that value already exists as a primary key in another table. This rule ensures that the relationship between tables remains valid and consistent.
E.g.:
CREATE TABLE Student
(
    RollNo INT PRIMARY KEY,
    Name VARCHAR(50)
);
The RollNo is the primary key of the Student table.
CREATE TABLE Course
(
    CourseNo INT PRIMARY KEY,
    RollNo INT,
    FOREIGN KEY (RollNo) REFERENCES Student(RollNo)
);
The CourseNo is the primary key of the Course table and RollNo is a foreign key that refers to Student(RollNo).
INSERT INTO Student VALUES (101, 'Suresh Kulahry'); /* Valid (Allowed) */
INSERT INTO Course VALUES (201, 101); /* Valid (Allowed) */
INSERT INTO Course VALUES (202, 105); /* Invalid (Not Allowed) */

[ Option C ]

A super key is a set of one or more attributes that uniquely identify each record in a table. However, a super key may include extra attributes that are not necessary for uniqueness.

When we remove all unnecessary attributes from a super key, leaving the minimal set of attributes that still uniquely identify each record, it becomes a candidate key.

Every candidate key is a minimal super key, meaning it cannot be reduced any further without losing the ability to uniquely identify records. From among the candidate keys, one is selected as the primary key to uniquely identify rows in the table.

[ Option C ]

A superkey is any set of attributes that can uniquely identify a tuple in a relation. If you have a relation with n attributes and one candidate key consisting of k attributes, the total number of super keys is 2^n−k.
Remember, this formula can only work with a relation has only one candidate key consisting of k attributes.
Given,
Relation: R(A, B, C)
Total Attributes (A,B,C) n = 3.
Candidate key B : k = 1
Total super keys: 2³⁻¹ = 2² = 4.

[ Option B ]

A super key is any set of attributes that contains the primary key or a superset of it. Since the primary key is VY, any super key must at least include both attributes V and Y.

(a)  VXYZ: Has V,X,Y,Z. It contains both V and Y. So, this is a super key.
(b)  VWXZ: Has V,W,X,Z. It contains V but does not have Y. So, this is not a super key.
(c)  VWXY: Has V,W,X,Y. It contains both V and Y. So, this is a super key.
(d)  VWXYZ: Has V,W,X,Y,Z. It contains both V and Y. So, this is a super key.

[ Option C ]

The entity integrity constraint states that a primary key cannot contain NULL values. This rule ensures that every record in a table can be uniquely identified.
In the case of a composite (more than one field) primary key, then none of those fields are allowed to be NULL.
E.g. (1):
CREATE TABLE Student
(
    Rollno INT PRIMARY KEY,
    Name VARCHAR(50)
);
Here, Rollno is the primary key and it cannot be NULL. Every student must have a valid Rollno.
E.g. (2):
CREATE TABLE Enrollment
(
    Rollno INT,
    Courseno INT,
    PRIMARY KEY (Rollno, Courseno)
);
Here, the primary key is composite, made of Rollno and Courseno. Both fields must have values. Neither Rollno nor Courseno can be NULL.
 

[ Option A ]

In a database table, a non-key field (non-key attribute) is a field that does not uniquely identify a record. It is not part of any candidate key and therefore cannot serve as a primary key.

[ Option A ]

In a Database Management System (DBMS), Keys are attributes or sets of attributes used to uniquely identify each tuples (rows) in a table (relation).
In a relational database, a Super Key is any set of attributes that uniquely identifies tuples, but it may contain extra attributes.
A Candidate Key is a minimal super key, meaning no subset of it can be a super key.

Among candidate keys, one is chosen as the Primary Key to uniquely identify tuples but it is not chosen based on the number of attributes. It is usually selected for simplicity, stability, and efficiency, not because it has the highest number of attributes.
So, statements I and II correctly describe Super Key and Candidate Key, while III is incorrect because the primary key is not selected based on attribute count.