Primary Vs. Candidate Key: Know the Difference Between Primary and Candidate Key
Both of these are different attributes that help users access and get records and tuples from the available tables. These keys also help users to establish a relationship between two or more different tables. There is a significant difference between primary and candidate keys, but both of these help a user uniquely identify records in an available table.
Let us know more about both of these in detail.
What is a Primary Key?
A primary key refers to a set of various attributes (or a single attribute) that help in uniquely identifying the tuples available in a table or a relation. A primary key is basically a minimal super key. Thus, a relationship consists of only one primary key.
What is a Candidate Key?
A candidate key also refers to a set of various attributes (or a single attribute) that help in uniquely identifying the tuples available in a table or a relation. The difference here is that a primary key acts as a minimal super key. Thus, a relation can only have one primary key. On the other hand, multiple candidate keys (two or more) can take place in any relationship. The attributes in a candidate key may contain a NULL value that opposes the primary key in it.
Any table can have more than one candidate key for identifying the tuples in a relation (uniquely).
Difference Between Primary and Candidate Key
Here is a list of the differences between Primary and Candidate Key.
| Parameters | Primary Key | Candidate Key |
| Meaning and Definition | It is a non-null and unique key that helps in identifying any record on the table uniquely. One table can only contain a single primary key. | It is also a unique type of key that helps in identifying any record on the table uniquely. The difference is that any table can have two or more candidate keys. |
| Objective | It is the most crucial part of any table or relation. It acts as the minimal super key for a table. | It basically signifies the key that we can use in the form of a primary key. |
| NULL | The column value of a primary key can never be NULL. | The columns in a candidate key can have a NULL value. |
| Correlation | A primary key is basically a type of candidate key. | A candidate key may or may not be a type of primary key. |
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