DBMS Keys: Primary, Super, Candidate, Composite and Foreign key
Key : A key is a data item that exclusively identifies a record. Example :Account_number, Employee_number and customer_number are used a key fields because they specifically identify a record stored in a database.
Super Key : A super key for an entity is a set of one or more attributes whose combined value uniquely identifies the entity in the entity set.
Primary Key : The primary key uniquely identifies each record in a table and must never be the same for two records.
Candidate Key : A candidate key is an attribute or set of attributes that uniquely identifies a record. These attributes or combinations of attributes are called candidate key.
Composite Key : A composite key is a group of fields that are combined together to uniquely identify a record. When we design a database, we will have tables that will use more than one column as a part of primary key. These keys are called composite key or concatenated key.
Foreign Key : In a relation, the column whose data values correspond to the values of a key column in another relation is called a Foreign Key.
Secondary Key : A secondary key is an attribute or combination of attributes that may not be a candidate key but classifies the entity set on a particular characteristic.