DAY - 5
Database Keys
Keys
are very important part of Relational database. They are used to establish and
identify relation between tables. They also ensure that each record within a
table can be uniquely identified by combination of one or more fields within a
table.
1.
Super Key
Super Key
is defined as a set of attributes within a table that uniquely identifies each
record within a table. Super Key is a superset of Candidate key.
2.
Candidate Key
Candidate keys
are defined as the set of fields from which primary key can be selected. It is an
attribute or set of attribute that can act as a primary key for a table to
uniquely identify each record in that table.
3.
Primary Key
Primary key is a
candidate key that is most appropriate to become main key of the table. It is a
key that uniquely identify each record in a table.
4.
Composite Key
Key that consist
of two or more attributes that uniquely identify an entity occurance is called Composite
key. But any attribute that makes up the Composite key is not a
simple key in its own.
5.
Secondary or Alternative key
The candidate key
which are not selected for primary key are known as secondary keys or
alternative keys
6.
Non-key Attribute
Non-key attributes are
attributes other than candidate key attributes in a table.
Non-prime Attribute
Non-prime Attributes are
attributes other than Primary attribute.
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