Primary Key

In database relational modeling and implementation, a unique key (also known as a candidate key) of a relation is a minimal superkey for that relation; that is, a set of attributes such that: In database relational modeling and implementation, a unique key (also known as a candidate key) of a relation is a minimal superkey for that relation; that is, a set of attributes such that: When a column or set of columns is defined as unique to the database management system, the system verifies that each set of values is unique before assigning the constraint. After the columns are defined as unique, an error will occur if an insertion is attempted with values that already exist. Some systems will not allow key values to be updated, all systems will not allow duplicates. This ensures that uniqueness is maintained in both the primary table and any relations that are later bound to it.