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In the context of a relational database, Keys are one of the basic
requirements of a relational database model. keys are fundamental
components that ensure data integrity, uniqueness, and efficient access.
It is widely used to identify the tuples(rows) uniquely in the table.
We also use keys to set up relations amongst various columns and tables
of a relational database. Let's explore the various types of keys used in a
relational model, which are essential for organizing and querying data
effectively.
Why do we require Keys in a DBMS?
Keys are crucial in aDatabase Management System (DBMS) for several
reasonS:
Uniqueness: Keys ensure that each record in a table is unique and
can be identified distinctly.
Data lntegrity: Keys prevent dataduplication and maintain the
consistency of the data.
Efficient Data Retrieval: By defining relationships between tables,
keys enable faster querying and better data organization. Without
keys, it would be extremely difficult to manage large datasets, and
queries would become inefficient and prone to errors.
Different Types of Database Keys
1. Super Key
The set of one or more attributes (columns) that can uniquely identify a
tuple (record) is known as Super Key. t may include extra attributes
that aren't essential for uniqueness but stilluniquely identify the row. For
Example, STUD_NO, (STUDNO, STUD_NAME), etc.
Asuper key is a group of single or multiple keys that uniquely
identifies rows in atable. It supports NULL values in
rows.
A super key can contain extra attributes that aren't
necessary for
uniqueness. For example, if the "STUDNO" column can uniquely
identify a student, adding "SNAME" to it will still form a valid
super key, though it's unnecessary.
Example: Consider the STUDENT table
STUD_NO SNAME ADDRESS PHONE
1 Shyam Delhi 123456789
2 Rakesh Kolkata 223365796
3 Suraj Delhi 175468965
Asuper key could be acombination of STUD_NO and PHONE, as this
combination uniquely identifies a student.
Super Key
Candidate Key
Primary Key
Relatio
n between Primary Key, Candidate Key, and Super Key
2. Candidate Key
The minimal set of attributes
that canuniquely identify atuple is known
as a candidate key. For Example, STUD NO in STUDENT
relation.
A candidate key is a minimal super key, meaning it can
uniqueiy
identify a record but contains no extra attributes.
" lt is a super key with no repeated data is called a
candidate key.
The minimalset of attributes that can uniquely identify
a record.
" Acandidate key must contain unique values,
ensuring that no two
rows have the same value in the candidate key's columns.
Every table must have at least a single candidate key.
A table can have multiple candidate keys but only one primary
key.
Example: For the STUDENT table below, STUD_NO can be a candidate
key, as it uniquely identifies each record.
STUD_NO SNAME ADDRESSPHONE
1 Shyam Delhi 12345%789
2 Rakesh Kolkata 223365796
3 Suraj Delhi 175468965
Table STUDENTCOURSE
STUD_NO TEACHER_NO COURSE_NO
STUD _NO TEACHERNO COURSE_NO
1 O01 CO01
2 COOS
A composite candidate key example: (STUD_NO, COURSE_NO} can be a
candidate key for a STUDENT_COURSE table.
3. Primary Key
There can be more than one candidate key in elation out of which one
can be chosen as the primary key. For Example, STUD_NO, as well as
STUD_PHONE, are candidate keys for relation STUDENT but STUD_NO
can be chosen as the primary key (only one out of many candidate keys).
Aprimary key is a unque key, meaning it can uniquely identify
each record (tuple) in a table.
It must have unique values and cannot contain any duplicate values.
A primary key cannot be NULL, as it needs to provide avalid,
unique identifier for every record.
Aprimary key does not have to consist of a single column. In some
cases, a composite primary key (made of multiple columns) can be
used to uniquely identify records in a table.
Databases typically store rows ordered in memory according to
primary key for fast access of records using primary key.
Example:
STUDENT table -> StudentSTUD NO, SNAME, ADDRESS, PHOWE) »
STUDNOis a primary key
Table STUDENT
STUD_NO SNAME ADDRESSPHONE
1
Shyam Delhi 123456789
2 Rakesh Kolkata 223365746
3 Suraj Delhi 175468965
4. Altemate Key
An alternate key is any candidate key in atable that is not chosen as the
primary key. In other words, all the keys that are not selected as the
primary key are considered alternate keys.
An alternate key is also referred to as a secondary key because it
Can uniquely identify records in a table, just like the primary key.
An alternate key can consist of one or more columns (fields) that
can uniquely identify a record, but it is not the primary key
Eq:- SNAME, and ADDRESS is Alternate keys
Example: In the STUDENT table, both STUD NO and PHONE are
candidate keys. If STUD NO ischosen as the primary key,
then PHONE Would be considered an alternate key.
Candidate Key
StudlDRoll No. First Name Last Name Email
21 Tom Cox abc@gfg.org
2 22 John Butler xyz@gfg.org
3 23 Alice Peterson mno@gfg.org
Primary Key
Alternate Key
Primary Key, Candidate Key, and Alternate Key
s. Foreign Key
A foreiqn key is an attribute in one table that refers to the
primary key
in another table. The table that contains the foreign key is called the
referencing table, and the table that is referenced is called the referenced
table.
A foreign key in one table points to the primary key in another
table, establishing a relationship between them.
" It helps connect two or more tables, enabling you to create
relationships between them. This is essential for maintaining data
integrity and preventing data redundancy.
They act as a cross-reference between the tables.
For example, DNO is a primary key in the DEPT table and a non
key in EMP
Example: Consider the STUDENTCOURSE table
STUD_NO TEACHER NO COURSE NO
1 O0S COO1
OS6 COOs
Here, STUD_NO in the STUDENT COURSE table is a foreign key that
references the STUD_NOprimary key in the sTUDENT table.
Explanation:
Unlike the Primary Key of any given relation, Foreign Key can be
NULL as wellas may contain duplicate tuples i.e. it need not follow
uniqueness constraint. For Example, STUD NO in the
STUDENT_COURSE relation is not unique.
It has been repeated for the first and third tuples. However, the
STUD_NO in STUDENT relation is a primary key and it needs to be
always unique, and it cannot be null.
Example: In the sTUDENT COURSE table, (STUD NO, COURSE NO} can form
a composite key to uniquely identify each record.
Candidate Key Foreign Key
Table 1 Table 2
Dld Designation
EmplDEmp Name EmpLicence EmpPassport DId
ELI01 PA123 2 2 BPO
001 Tom
EL102 PA125 3 3 Account
005 John
EL103 PAI29 5 IT
O08 Alice
Primary Key
Alternate Key Primary Key Candidate Key Alternate Key
Superkey
Unique Key
