The document provides an overview of keys in database relations, explaining their definitions and types including primary, candidate, alternate, super, composite, foreign, and unique keys. It illustrates how primary and foreign keys function within student and course relation examples, highlighting the flexibility in allowing nullable foreign keys. The document also outlines the purpose, uniqueness, null value handling, and index usage associated with each key type.

1. Prepared by
Dr. Arun Kumar Singh
Dr. Arun K Singh arun.k.singh.iiit@gmail.com
Keys_Arun Sir 1
Keys

2. KEYS:
• A Key is an attribute or a set of attributes in a relation that
identifies a tuple (record) in a relation.
• The keys are defined in a table to access or sequence the
stored data quickly and smoothly.
• They are also used to create relationship between different
tables
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3. Type of KEYS:
•1.Primary Key
•2. Candidate Key
•3. Alternate Key
•4. Super Key
•5. Composite Key
•6. Foreign Key
•7. Unique Key
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4. PRIMARY KEYS:
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5. Condition: Primary Key
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6. Candidate Key
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7. Alternate Key
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8. Super Key
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9. Composite Key
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10. Foreign Key
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11. Example: Students and Courses with Nullable Foreign Key
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Courses Relation
Relation Name: Courses
Primary Key: Course ID
Students Relation
Relation Name: Students
In this case, we allow the course_id to be nullable, indicating that a
student might not be enrolled in any course.
Primary Key: student_id
Foreign Key: course_id (references Courses(course_id))
A NULL value in the course_id column indicates that the student is not currently enrolled in any course. This flexibility allows the
table to accommodate students who may not have selected a course yet.

12. Unique Key
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13. Example:
Consider the following Student schema-
• Student ( roll , name , sex , age , address , class , section )
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Given below are the examples of candidate keys-
( class , section , roll )
( name , address )
These are candidate keys because each set consists of minimal attributes required to identify each student
uniquely in the Student table.

14. Candidate Key Example
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For example, employee relation has two attributes, Employee_Id and PAN_No, that act
as candidate keys. In this relation, Employee_Id is chosen as the primary key, so the other
candidate key, PAN_No, acts as the Alternate key.

15. Key Type Definition Uniqueness Null Values Purpose Index Usage Alteration
Primary Key
Uniquely
identifies each
record in a
table.
Required Not Allowed
Data integrity,
record
identification.
Creates Unique
Index
Within same
table
May be
complex,
impact other
tables.
Foreign Key
Establishes a
relationship
between tables.
Not Required Allowed
Data
consistency,
relationship
maintenance.
May create
Index
Between tables
More flexible
for
maintenance.
Candidate Key
Alternative
unique keys that
could be
primary keys.
Required Not Allowed
Backup primary
key options.
May create
Index
Within same
table
May become
primary key.
Super Key
Combination of
attributes that
uniquely
identify.
Required Allowed
Conceptual, not
enforced.
May create
Index
Conceptual use N/A
Composite Key
Combined
attributes used
as a single key.
Required Not Allowed
Specialized
unique
identification.
Creates
Composite
Index
Within same
table
May be
complex,
impact
performance.
Unique Key
Ensures
column(s) have
unique values.
Required Allowed
Enforce
uniqueness, no
primary key.
Creates Unique
Index
Within same
table
May be used as
primary key.
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16. Dr. Arun K Singh arun.k.singh.iiit@gmail.com
Keys_Arun Sir 16