What is a candidate key? How does it relate to a primary key?
Posted: Tue May 20, 2025 10:41 am
In the context of relational databases, a candidate key is a column or a set of columns that can uniquely identify each row in a table. It possesses two crucial properties:
Uniqueness: Every value in the candidate key column(s) must be unique across all rows in the table. No two rows can have the same value for the candidate key.
Minimality (Irreducible): No proper subset of the candidate key can uniquely identify a row. In other words, you cannot remove any column from the candidate key and still maintain its uniqueness. If you can remove a column and still uniquely identify rows, then the original set of columns was a "superkey" but not a "candidate key" (which is a minimal superkey).
How does it relate to a Primary Key?
The relationship between a candidate key and a primary key is hierarchical and crucial to database design:
All primary keys are candidate keys, but not all candidate azerbaijan number database keys are primary keys.
Think of it this way:
Identifying all potential unique identifiers: When you design a database table, you might find several columns or combinations of columns that could, in theory, uniquely identify each record. These are all your candidate keys.
Example: Consider a Students table with columns: StudentID, NationalID, EmailAddress, FirstName, LastName, ``DateOfBirth`.
StudentID is likely unique for each student.
NationalID (like a Social Security Number or national identification number) is also designed to be unique for each person.
EmailAddress might also be unique for each student (assuming one email per student).
A combination of FirstName, LastName, and DateOfBirth might be unique, but it's less reliable and not minimal (you probably don't need all three to uniquely identify everyone).
In this example, StudentID, NationalID, and EmailAddress are all strong candidate keys because each can uniquely identify a student, and they are generally minimal.
Choosing the "Best" Candidate Key as the Primary Key: From the set of identified candidate keys, the database designer then chooses one to be the primary key for that table. This choice is typically based on several factors:
Stability: The primary key should ideally be a value that rarely, if ever, changes. For example, a StudentID assigned by an institution is usually stable, whereas an EmailAddress might change if the student switches providers.
Simplicity/Brevity: A shorter, simpler key (e.g., a single integer ID) is often preferred over a long, complex key (e.g., a composite key with multiple text fields) for performance and ease of use.
Meaningfulness (or lack thereof): Sometimes, a "surrogate key" (an artificially generated, meaningless ID like an auto-incrementing integer) is chosen as the primary key even if natural candidate keys exist. This is because natural keys can sometimes change or have business meaning that could complicate things.
Non-Nullability: While some database systems might allow NULLs in candidate keys (though it's generally avoided in practice for any key meant for unique identification), a primary key must not contain NULL values. This is a fundamental constraint of a primary key.
Once a primary key is chosen, the other candidate keys that were not selected as the primary key are often referred to as alternate keys. These alternate keys can still have unique constraints applied to them to ensure data integrity, but they are not the "official" unique identifier for the table that foreign keys would typically reference.
In summary, candidate keys are the pool of potential unique identifiers for a table. The primary key is the specific candidate key that is officially designated to uniquely identify each row and serve as the main mechanism for establishing relationships with other tables.
Uniqueness: Every value in the candidate key column(s) must be unique across all rows in the table. No two rows can have the same value for the candidate key.
Minimality (Irreducible): No proper subset of the candidate key can uniquely identify a row. In other words, you cannot remove any column from the candidate key and still maintain its uniqueness. If you can remove a column and still uniquely identify rows, then the original set of columns was a "superkey" but not a "candidate key" (which is a minimal superkey).
How does it relate to a Primary Key?
The relationship between a candidate key and a primary key is hierarchical and crucial to database design:
All primary keys are candidate keys, but not all candidate azerbaijan number database keys are primary keys.
Think of it this way:
Identifying all potential unique identifiers: When you design a database table, you might find several columns or combinations of columns that could, in theory, uniquely identify each record. These are all your candidate keys.
Example: Consider a Students table with columns: StudentID, NationalID, EmailAddress, FirstName, LastName, ``DateOfBirth`.
StudentID is likely unique for each student.
NationalID (like a Social Security Number or national identification number) is also designed to be unique for each person.
EmailAddress might also be unique for each student (assuming one email per student).
A combination of FirstName, LastName, and DateOfBirth might be unique, but it's less reliable and not minimal (you probably don't need all three to uniquely identify everyone).
In this example, StudentID, NationalID, and EmailAddress are all strong candidate keys because each can uniquely identify a student, and they are generally minimal.
Choosing the "Best" Candidate Key as the Primary Key: From the set of identified candidate keys, the database designer then chooses one to be the primary key for that table. This choice is typically based on several factors:
Stability: The primary key should ideally be a value that rarely, if ever, changes. For example, a StudentID assigned by an institution is usually stable, whereas an EmailAddress might change if the student switches providers.
Simplicity/Brevity: A shorter, simpler key (e.g., a single integer ID) is often preferred over a long, complex key (e.g., a composite key with multiple text fields) for performance and ease of use.
Meaningfulness (or lack thereof): Sometimes, a "surrogate key" (an artificially generated, meaningless ID like an auto-incrementing integer) is chosen as the primary key even if natural candidate keys exist. This is because natural keys can sometimes change or have business meaning that could complicate things.
Non-Nullability: While some database systems might allow NULLs in candidate keys (though it's generally avoided in practice for any key meant for unique identification), a primary key must not contain NULL values. This is a fundamental constraint of a primary key.
Once a primary key is chosen, the other candidate keys that were not selected as the primary key are often referred to as alternate keys. These alternate keys can still have unique constraints applied to them to ensure data integrity, but they are not the "official" unique identifier for the table that foreign keys would typically reference.
In summary, candidate keys are the pool of potential unique identifiers for a table. The primary key is the specific candidate key that is officially designated to uniquely identify each row and serve as the main mechanism for establishing relationships with other tables.