DBMS Keys : Primary, Super, Candidate, Composite and Foreign 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. Super set of any candidate key is also known as Super key.
Here Four Super Keys are present in this table is given below:
Primary Key :
The primary key uniquely identifies each record in a table and must never be the same for two records.
Here only one Primary Key is present in this table is given below:
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 keys.
Here Two Candidate Keys are present in this table is given below:
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 the primary key. These keys are called composite key or concatenated key.
Here Two Composite Keys are present in this table is given below:
Foreign Key :
In 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.