What are the difference between superkey and candidate key?

Answer 1

A superkey is defined as a set of attributes of a relation for which it holds that in all relations assigned to

that variable there are no two distinct tuples (rows) that have the same values for the attributes in this set.

As an example

Code:

Roll Number | First Name | Last Name

CSU0001 | Shabbir | Bhimani |

CSU0002 | SomeName | SurName |

CSU0003 | Larry | page |

Now here we have the following as super keys

1. Roll Number | First Name

2. Roll Number | First Name | Last Name

Now in plain language Any Unique key with some Non Unique key combination is called a super key of the

relationship.

A candidate key of a relationship is a set of attributes of that relationship such that there are no two distinct

tuples with the same values for these attributes. In simple example candidate key is a minimal superkey, i.e. a

superkey of which no proper subset is also a superkey.

Since a relation is a set(no duplicate elements), it holds that every relation will have at least one candidate key

(because the entire heading is always a superkey). For practical reasons RDBMSs usually require that for each

relation one of its candidate keys is declared as the primary key

Quote:

For example, Given an employee table consisting of the columns:

employeeID

name

job and

departmentID

we could use the employeeID in combination with any or all other columns of this table to uniquely identify a row

in the table. Examples of superkeys in this table would be {employeeID, Name}, {employeeID, Name, job}, and

{employeeID, Name, job, departmentID}.

In a real database we don't need values for all of those columns to identify a row. We only need, per our example,

the set {employeeID}. This is a minimal superkey - that is, a minimal set of columns that can be used to identify a

single row. So, employeeID is a candidate key.

Now, if employeeID is a candidate key then why not it is the superkey. Because employeeID can also uniquely

identify the tuples.

(2) In your example why Roll number is not the superkey as it is uniquely identifying the tuples?

(3)

Quote:

Any Unique key with some Non Unique key combination is called a super key of the relationship

Is it necessary that Unique key has to be combined with some Non Unique key to be called as a super key.

Unique key is also a super key but the minimal super key is called candidate key and all candidate keys are super

keys but the reverse is not true.