Candidate Key: Definition, Examples


A candidate key is a set of one or more fields/columns that can uniquely identify a record in a database table. It’s a type of unique key and each table has at least one candidate key. From these candidate keys, one is selected as a primary key, which serves as the principal means to identify and relate records in the table.


The phonetics of the keyword “Candidate Key” is: /ˈkændɪdət kiː/

Key Takeaways

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  1. A Candidate Key is a set of one or more fields/columns that can identify a record uniquely in a table. It is a unique identifier for each row within a database table.
  2. There can be more than one Candidate Key in a table. Each can be a Primary Key, but only one Candidate Key becomes the Primary Key.
  3. Candidate Keys enforce integrity constraints, ensuring that the data within the records remains reliable and consistent throughout the life of the database.

“`These points elaborate on the fundamental principles of Candidate Keys in a database system.


Candidate Key is an essential term in the technology world, particularly within relational database management systems, as it represents the fundamental aspect of ensuring data is uniquely identifiable within a database. In database design, a Candidate Key serves as an identifier for each row of data by having a unique concatenation of specific fields. Such uniqueness ensures there’s no duplication, promoting data consistency and integrity. Furthermore, a Candidate Key is a potential Primary Key, a critical component dictating how tables in a database relate to one another. Therefore, understanding and correctly implementing Candidate Keys supports the development of accurate, efficient, and reliable database systems.


A Candidate Key is an essential component in the realm of relational databases. The primary purpose of a candidate key is to identify unique rows of data within a table. It serves as a unique identifier, ensuring that no two records share the same value. This distinctive feature helps maintain the integrity of data within a relational database, eliminating the possibility of duplicate entries and enhancing search performance within a large dataset. Moreover, a Candidate Key can also form a basis for the primary key. In any relational database, there may be multiple candidate keys, but there’s always only one primary key. The database admin can choose one of the Candidate Keys to be a primary key depending on the specific use case. Ultimately, the usage of the Candidate Key is instrumental in managing, organizing, and retrieving data efficiently.


1. Student database in a School: In a school database, the student identification number (also called roll number or unique student ID) can be an example of a candidate key. This ID is unique to every student. Other potential candidate keys could be a government issued id like social security number (in U.S.) or Aadhar number (in India). 2. Employee database in a Company: In company databases, an employee ID number is a good example of a candidate key. It is unique to each employee and is specifically created to track employee records. Another potential candidate key would be an email address of an employee, as it is also unique for every individual in a business environment.3. Customer database in an E-commerce platform: On an e-commerce website, the customer database might use the customer’s unique user ID or email id as a candidate key. Other possibilities could be the combination of the customer’s phone number and birth date. Remember that the deciding factor for a candidate key is that it must be unique and must have the ability to identify a row or record uniquely in a relational database.

Frequently Asked Questions(FAQ)

Q1: What is a Candidate Key in technology terms?A1: In the context of databases, a Candidate Key is an attribute or set of attributes that can uniquely identify a record in a relational database. It means every row in the database can be uniquely identified by the Candidate Key.Q2: How many Candidate Keys can a table have?A2: A table can have one or several Candidate Keys. These are keys that could potentially be chosen as the Primary Key.Q3: How does a Candidate Key differ from a Primary Key?A3: A Candidate Key differs from a Primary Key in that it is not necessarily chosen to be the main means of identifying records within a table. While a table can have multiple Candidate Keys, it can only have one Primary Key.Q4: Can a Candidate Key contain NULL values?A4: No, a Candidate Key cannot contain NULL values. It must be able to uniquely and fully identify records within the table, which a NULL value would prevent.Q5: How are Candidate Keys chosen?A5: Candidate Keys are chosen based on their ability to uniquely identify each row within the table. They should be minimal, meaning the key should not have any unnecessary attributes.Q6: What happens if there are multiple Candidate Keys?A6: If there are multiple Candidate Keys, any one of them can be chosen to be the Primary Key. The remaining keys will be alternate keys.Q7: What is the importance of a Candidate Key?A7: Candidate Keys are important because they ensure integrity and accuracy within a database. They help to avoid duplicate entries and ensure each record can be uniquely identified. Q8: What is an example of a Candidate Key?A8: An example of a Candidate Key could be the social security number in a table storing personal details of individuals. As each person has a unique social security number, it can serve as a Candidate Key. Q9: Can a Candidate Key be a compound key?A9: Yes, a Candidate Key can be a compound key. A compound key is a key that consists of more than one attribute to ensure uniqueness in a table.Q10: Is it necessary to have a Candidate Key in every database?A10: Yes, it’s necessary to have at least one Candidate Key in every table of a relational database. Without a Candidate Key, it wouldn’t be possible to uniquely identify records, which is required for performing specific operations and maintaining data integrity.

Related Finance Terms

  • Primary Key
  • Foreign Key
  • Super Key
  • Unique Key
  • Composite Key

Sources for More Information

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