Primary key and unique key are both types of constraints used in database management systems to enforce the uniqueness of values in a table. However, there are several important differences between them:
1. Uniqueness:
- Primary Key: A primary key uniquely identifies each record in a table. No two rows can have the same value for any column that is part of the primary key. It cannot contain NULL values.
- Unique Key: A unique key ensures that all values in a column (or a combination of columns) are unique across the table, but it can allow one NULL value (in most relational databases). This means that while the column(s) cannot contain duplicate values, a NULL value is permitted.
2. Number of Constraints:
- Primary Key: A table can have only one primary key constraint. This primary key can consist of one or more columns (composite key).
- Unique Key: A table can have multiple unique key constraints. Each of these unique keys can specify different columns or sets of columns to enforce uniqueness.
3. Default Index:
- Primary Key: When you define a primary key on a table, it automatically creates a unique index on that key to enforce uniqueness and improve performance on lookups based on that key.
- Unique Key: A unique key also creates a unique index to enforce uniqueness; however, it does not provide the same guarantee of being the primary means of identifying records in the table.
4. Semantic Significance:
- Primary Key: The primary key is often considered the main identifier for records in a table. It plays a crucial role in establishing relationships between tables, especially in foreign key constraints.
- Unique Key: A unique key may be used to enforce business rules regarding data integrity (e.g., an employee's email address must be unique across the organization) but is not intended to serve as the primary identifier for the records.
5. Usage in Foreign Keys:
- Primary Key: Other tables can reference the primary key of a table by establishing a foreign key relationship, thereby enforcing referential integrity.
- Unique Key: Other tables can also reference a unique key as a foreign key relationship, providing more flexibility in table design.
In summary, both primary keys and unique keys enforce uniqueness within a table, but they differ in their constraints on NULL values, the number of instances allowed, and their significance in table relationships.