Primary Key in DBMS
Primary Key in DBMS: In the context of Database Management Systems (DBMS), keys are crucial for defining the structure of a database and ensuring data integrity. Here’s an explanation of the Primary Key in DBMS, Super Key, and Candidate Key with an example:
Super Key
A Super Key is any set of one or more columns that can uniquely identify a row in a table. This means that no two rows can have the same values for the columns that make up the Super Key.
Candidate Key
A Candidate Key is a minimal Super Key, meaning it is a Super Key with no redundant attributes. In other words, a Candidate Key is a Super Key such that if any attribute is removed, it will no longer be a Super Key. A table can have multiple Candidate Keys.
Primary Key in DBMS
A Primary Key is a special Candidate Key chosen by the database designer to uniquely identify each row in the table. There can only be one Primary Key per table, and it cannot contain NULL values.
Example
Consider a table Students with the following columns:
StudentID(unique identifier for each student)Email(unique email address of each student)Name(name of the student)PhoneNumber(unique phone number of each student)
Here’s how these keys would work:
Super Key
Any combination of columns that can uniquely identify a row can be a Super Key. Examples include:
- {StudentID}
- {Email}
- {PhoneNumber}
- {StudentID, Email}
- {StudentID, PhoneNumber}
- {Email, PhoneNumber}
- {StudentID, Email, PhoneNumber}
And so on. Any set of columns that can uniquely identify each student can be considered a Super Key.
Candidate Key
From the Super Keys, we need to find the minimal sets, which means removing any redundant attributes. Examples of Candidate Keys are:
- {StudentID}
- {Email}
- {PhoneNumber}
Each of these keys can uniquely identify a student and is minimal, meaning no subset of these keys can also uniquely identify a student.
Primary Key in DBMS
The Primary Key is selected from the Candidate Keys. Typically, the choice is based on which key is most suitable and less likely to change. In this example, StudentID is often chosen as the Primary Key because it is a unique, immutable identifier. Therefore, the Primary Key for the Students table would be:
- {StudentID}
Summary
- Super Key: A set of one or more columns that can uniquely identify a row (e.g.,
{StudentID},{Email},{StudentID, Email}). - Candidate Key: A minimal Super Key (e.g.,
{StudentID},{Email},{PhoneNumber}). - Primary Key in DBMS: The selected Candidate Key that uniquely identifies each row and cannot be NULL (e.g.,
{StudentID}).
By understanding these key concepts, you can ensure data integrity and proper database design in DBMS.
