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What is the difference between primary key and candidate key?

The primary key and candidate key are both concepts used in database design to uniquely identify rows in a table. However, there are some differences between them:

Primary Key:

  1. A primary key is a special type of candidate key that is chosen to uniquely identify each row in a table.
  2. In a table, there can be only one primary key.
  3. The primary key must be unique and non-null for each row.
  4. It provides a means to identify and access individual rows efficiently.
  5. By default, a primary key constraint enforces both uniqueness and non-nullability.

Candidate Key:

  1. A candidate key is a set of one or more columns that can uniquely identify a row in a table.
  2. A table can have multiple candidate keys.
  3. Each candidate key must be unique and non-null for each row.
  4. It represents a potential choice for the primary key of a table.
  5. Candidate keys help in identifying and distinguishing rows.
  6. A candidate key may or may not be selected as the primary key based on factors such as data dependencies, business rules, and design considerations.

In summary, a candidate key is a set of columns that can uniquely identify a row, whereas a primary key is the chosen candidate key that uniquely identifies each row in a table. The primary key is the main identifier for a table, while candidate keys are alternative choices for identifying rows.

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