What is the Difference Between Primary Key and Candidate Key?
🆚 Go to Comparative Table 🆚The primary key and candidate key are both used to uniquely identify records in a table, but they have some differences:
- Number of keys: A table can have only one primary key, while it can have one or more candidate keys.
- NULL values: Candidate keys can contain NULL values, unlike primary keys, which must be non-null and unique.
- Minimal super key: A primary key is a minimal super key, meaning it is a key that cannot be reduced any further without losing the ability to uniquely identify a record.
In summary, both primary keys and candidate keys are used to uniquely identify records in a table, but a primary key is a non-null, unique key that can be a minimal super key, while a candidate key can have NULL values and there can be multiple candidate keys in a table.
Comparative Table: Primary Key vs Candidate Key
Here is a table comparing the differences between primary keys and candidate keys:
| Feature | Primary Key | Candidate Key |
|---|---|---|
| Uniqueness | Identifies each record uniquely | Identifies each record uniquely |
| Minimal Super Key | Yes, it is a minimal super key | Yes, it can be a super key but not necessarily a minimal super key |
| Null Values | Cannot contain NULL values | Can contain NULL values |
| Number of Keys | Each table can have only one primary key | Each table can have multiple candidate keys |
| Selection | Chosen among candidate keys to maintain uniqueness in a table | Selected from the set of super keys, ensuring it does not have any redundant attribute |
| Examples | In a table 'Employee': EmployeeID, SSN are candidate keys. You can choose either EmployeeID or SSN as the primary key. | In a table 'Employee': EmployeeID & SSN are candidate keys. Any one of them can be chosen as the primary key. |
In summary, primary keys are unique and non-null attributes that identify each record in a table, and only one primary key can exist in a table. On the other hand, candidate keys are also unique attributes that identify each record in a table, but multiple candidate keys can exist in a table, and one of them is chosen as the primary key.
- Primary key vs Unique key
- Foreign key vs Primary key
- Applicant vs Candidate
- Primary vs General Election
- Public Key vs Private Key
- Open vs Closed Primary
- Primary vs Secondary Data
- Primary Partition vs Logical Partition
- Identifier vs Keyword
- Primary vs Secondary Memory
- Symmetric Key Encryption vs Public Key Encryption
- Primary Research vs Secondary Research
- Criteria vs Criterion
- Candidate Gene vs GWAS
- Primary Partition vs Extended Partition
- Primary vs Secondary Markets
- Dichotomous Key vs Taxonomic Key
- Primary Education vs Secondary Education
- Prime Minister vs Chief Minister