Keys

• Key is the attribute to search a record in the database.

• It is used to establish and identify relationships between tables.

• It identify any record or row of data uniquely.

Types of Keys

• Super Key

• Candidate Key

• Primary Key

• Composite Key

• Foreign Key

• Secondary or Alternative key

• Non-key Attributes

• Non-prime Attributes

Super Key

• Super Key A set of attributes within a table that can uniquely identify each record in a table.

• Super Key is a super-set of Candidate key.

Candidate Key

• Candidate Key A set of attribute which can uniquely identify each record in a table.

• A candidate key can never be NULL or empty.

• Its value should be unique.

• There can be more than one candidate keys for a table.

• A candidate key can be a combination of more than one attribute.

Primary Key

• Primary Key is the most appropriate candidate key that can uniquely identify each record in a table.

Composite Key

• Composite Key is consists of two or more attributes that uniquely identify any record in a table.

• But the attributes which together form the Composite key are not a key independently or individually.

Foreign Key

• Foreign keys are the attribute of the table which is used to point to the primary key of another table.

Secondary or Alternative key

• The candidate key which are not selected as primary key.

Non-key Attributes

• Attributes or fields of a table, other than candidate key in a table.

Non-prime Attributes

• Attributes other than Primary Key.