![]() It is a unique and non-null key to identify each table's records in a schema uniquely. The following comparison chart explains their main differences in a quick manner: Comparison Basis Each primary key can be a candidate key, but vice-versa is not possible.The candidate key can be NULL unless the attribute constraint is specified as not-null. This feature helps in finding the records in a referenced relation. We can also use the primary key as a foreign key in other relations. The primary key attribute can never be a NULL because its main function is to identify a record in a table or relation uniquely.It is not mandatory to define a primary key, but there cannot be a relationship without the candidate key.The fundamental difference among both keys is that a table or relation in a schema can have only one primary key, but there can be multiple candidate keys.In contrast, the candidate keys have many candidates, among which one candidate key can be selected as a primary key. The primary key is a unique and essential attribute of a table or relation.The following points explain the main differences between primary and candidate keys: Key differences between Primary and Candidate Key The rule to become a primary key among candidate keys is that the key's attribute values must be unique and can never be Null for any domain. ![]() Note that each candidate key can be a primary key, but only one should be chosen as the primary key. Since a table or relation can have more than one candidate key, one candidate key can qualify to become a primary key out of all candidate keys. Hence, it clarifies that more than one candidate key is available to identify the table or relation uniquely. Here we can figure out two candidate keys that are. For example, suppose we have a table named students with ID, Name, DOB, Age, and Address columns. This key can store a NULL value that opposes in a primary key. ![]() What is Candidate Key?Ī candidate key is an attribute or a set of attributes that identify each record in a table or relation uniquely but noted that a table could contain multiple candidate keys. Here we can make only the ID column a primary key because all other column values can be the same, but the ID column can't be the same. For example, suppose we have a table named students with ID, Name, Age, and Address columns. It is a minimal super key that's why it can contain only one primary key in any relationship. The primary key is a unique or not-null key that uniquely identifies each record in a table or relation. Before making a comparison, we will discuss in brief about these keys. In this article, we are going to compare essential differences between Primary and Candidate Keys based on various parameters. ![]() The most popular difference among them is that a table can have only one primary key but can have more than one candidate key. Both Primary and Candidate Key identifies each record uniquely in a table or relation. They are also used to construct a relationship between two tables. Keys in MySQL are an attribute or a set of attributes used to access records from tables. Next → ← prev Difference between Primary Key and Candidate Key
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