- Primary key cannot have a NULL value.
- Each table can have only one primary key.
- By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.
Composite Primary Key
- A Composite primary key is a set of columns whose values uniquely identify every row in a table. For example, in the table, if "Employee_ID" and "Employee Name" together uniquely identifies a row its called a Composite Primary Key
- Unique Constraint may have a NULL value.
- Each table can have more than one Unique Constraint.
- By default, Unique key is a unique non-clustered index.
- Foreign key in the table is primary key in another table.
- We can have more than one foreign key in a table.