Key
is a sole or mixture of multiple fields in a table. It is used to fetch records/data-rows
from data table according to the requirement. Keys are also used to create
relationship among different database tables or views.
Types
of SQL Keys
We
have following types of keys in SQL which are used to fetch records from tables
and to make relationship among tables or views.
1. Super Key
Super key is a set of one or
more than one keys that can be used to identify a record uniquely in a table.
2. Candidate Key
A Candidate Key is a set of
one or more fields/columns that can identify a record uniquely in a table.
There can be multiple Candidate Keys in one table. Each Candidate Key can work
as Primary Key.
3. Primary Key
Primary key is a set of one
or more fields/columns of a table that uniquely identify a record in database
table. It cannot accept null, duplicate values. Only one Candidate Key can be
Primary Key.
4. Alternate key
A Alternate key is a key
that can be work as a primary key. Basically it is a candidate key that
currently is not primary key.
5. Composite/Compound Key
Composite Key is a
combination of more than one fields/columns of a table. It can be a Candidate
key, Primary key.
6. Unique Key
Unique key is a set of one
or more fields/columns of a table that uniquely identify a record in database
table. It is like Primary key but it can accept only one null value and it cannot
have duplicate values.
7. Foreign Key
Foreign Key is a field in
database table that is Primary key in another table. It can accept multiple
null, duplicate values.
