Define Primary Key, Candidate Key and Super Key?

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 super-set of Candidate key.

Example -

Suppose we have a table that holds all the managers in a company, and that table is called Managers. The table has columns called ManagerID, Name, Title, and DepartmentID. Every manager has his/her own ManagerID, so that value is always unique in each and every row.

This means that if we combine the ManagerID column value for any given row with any other column value, then we will have a unique set of values. So, for the combinations of (ManagerID, Name), (ManagerID, TItle), (ManagerID, DepartmentID), (ManagerID, Name, DepartmentID), etc – there will be no two rows in the table that share the exact same combination of values, because the ManagerID will always be unique and different for each row. This means that pairing the Manager ID with any other column(s) will ensure that the combination will also be unique across all rows in the table.

And that is exactly what defines a superkey – it’s any combination of column(s) for which that combination of values will be unique across all rows in a table. So, all of those combinations of columns in the Manager table that we gave earlier would be considered to be superkeys. Even the ManagerID column is considered to be a superkey, although a special type of superkey as you can read more about below.

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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.

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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.

For example, An employee number uniquely identifies a member of staff within a company. An IP address uniquely addresses a PC on the Internet.

A primary key is mandatory. That is, each entity occurrence must have a value for its primary key.