Primary Key vs. Candidate Key: What's the Difference?
Edited by Aimie Carlson || By Janet White || Published on February 26, 2024
A primary key is a unique identifier for each record in a database table, while a candidate key is a field or set of fields that can uniquely identify records, with one becoming the primary key.
Key Differences
A primary key in a database table is a specific column or set of columns selected to uniquely identify each row. Candidate keys are columns or combinations of columns that are potential candidates for being the primary key. Each table can have only one primary key, but several candidate keys from which the primary key is chosen.
The primary key is chosen from among the candidate keys, and once set, it cannot have null values and must contain unique values. Candidate keys, before being chosen as the primary key, can have multiple potential keys that could serve as unique identifiers. The primary key is an actual key used for identifying records, while candidate keys are possible choices for the primary key.
Primary keys ensure the uniqueness of records and are used to establish relationships with other tables. Candidate keys, on the other hand, are identified when designing a database and represent all possible ways to uniquely identify a record in a table. The primary key's role is integral in database relationships, while candidate keys are more about database design and integrity.
In terms of constraints, the primary key is a constraint applied to a column or a set of columns in a table to uniquely identify each row. Candidate keys are not constraints themselves but are identified as potential primary keys. Once a candidate key is chosen as the primary key, it adopts the constraints and responsibilities of ensuring record uniqueness and cannot be null.
Every primary key is a candidate key, but not every candidate key becomes a primary key. The primary key is essentially the selected candidate key that has been employed for uniqueness and indexing in a database. Candidate keys are like nominees for a position, where the primary key is the one that ultimately fulfills the role.
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Comparison Chart
Definition
A unique identifier for each record in a table
Keys that can uniquely identify each record
Uniqueness
Must be unique
Must be unique
Null Values
Cannot have null values
Can have null values until chosen as primary
Number per Table
Only one primary key per table
Multiple candidate keys possible
Role in Relationships
Used to establish relationships with other tables
Considered during database design
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Primary Key and Candidate Key Definitions
Primary Key
A primary key uniquely identifies each row in a table.
In the database, the 'SerialNumber' column is the primary key for the products table.
Candidate Key
A candidate key is a column, or set of columns, that can uniquely identify records in a table.
Both 'EmailAddress' and 'PhoneNumber' are candidate keys for the customers' table.
Primary Key
A primary key is used to create relationships between tables.
The 'UserID' primary key in the Users table is used to link to the Orders table.
Candidate Key
A table can have multiple candidate keys.
For the books table, 'ISBN' and 'Title-Author' are both candidate keys.
Primary Key
A primary key cannot have duplicate or null values.
The primary key 'OrderID' ensures that each order has a unique identifier.
Candidate Key
One of the candidate keys is chosen as the primary key.
From the several candidate keys available, 'EmployeeNumber' was selected as the primary key.
Primary Key
A primary key is a special type of candidate key.
Among the candidate keys, 'LicensePlateNumber' was chosen as the primary key for the vehicle registration database.
Candidate Key
Candidate keys are potential primary keys.
In the initial design, 'Username' and 'Email' were considered as candidate keys for the account table.
Primary Key
A primary key is a unique identifier of each record in a database table.
The 'EmployeeID' field serves as the primary key in the employee database.
Candidate Key
Candidate keys can uniquely identify each row in a table.
The columns 'PassportNumber' and 'NationalID' are candidate keys for the citizens database.
FAQs
What is a primary key in a database?
A primary key is a unique identifier for each record in a database table.
Can a table have multiple candidate keys?
Yes, a table can have multiple candidate keys.
What is a candidate key?
A candidate key is a field or set of fields that can uniquely identify each record in a table.
Are primary keys always chosen from candidate keys?
Yes, primary keys are selected from the set of candidate keys.
How many primary keys can a table have?
A table can have only one primary key.
Can a candidate key become a primary key?
Yes, any candidate key can potentially become the primary key.
Is it mandatory to have a primary key in every table?
It's highly recommended but not mandatory to have a primary key for data integrity and relational purposes.
Why are candidate keys important in database design?
Candidate keys are important for identifying all possible ways to ensure record uniqueness in a table.
Can a primary key be changed once set?
Changing a primary key is possible but not advised due to potential data integrity issues.
What happens to the other candidate keys when one becomes the primary key?
The other candidate keys remain as alternative unique identifiers but are not used as primary keys.
Why is the primary key used in database relationships?
The primary key is used to establish and maintain relationships between different tables in a database.
How is a primary key different from a foreign key?
A primary key uniquely identifies records within its table, while a foreign key refers to a primary key in another table.
What are the characteristics of a good candidate key?
A good candidate key is minimal (fewest fields possible), unique, and stable (doesn't change frequently).
Do all candidate keys have the same constraints as primary keys?
No, candidate keys don't have the same constraints unless one becomes a primary key.
What happens if a database table doesn't have a primary key?
Without a primary key, data integrity and efficient data retrieval may be compromised.
Can a primary key contain null values?
No, a primary key cannot contain null values.
Is it possible to have a primary key with multiple candidate keys?
Yes, it's common to have a primary key chosen from multiple candidate keys.
Can a primary key be a combination of more than one column?
Yes, a primary key can be a composite key, formed by combining more than one column.
Can foreign keys and candidate keys be the same?
A foreign key can be a candidate key in its own table, but it primarily references a primary key in another table.
How does a primary key enhance data retrieval?
A primary key allows for efficient indexing and faster retrieval of specific records.
About Author
Written by
Janet WhiteJanet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie CarlsonAimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.
