Re: What are the differences between the terms, CANDIDATE KEY, PRIMARY KEY, SUPER KEY, COMPOSITE KEY?

Mark Johnson <102334.12_at_compuserve.com> wrote:

>"Abhishek" <abhisheksgumadi_at_gmail.com> wrote:

>>a) Any attribute or a collection of attributes which uniquely
>>identifies the tuples in a relation is called a canditate key.

>A superkey can be said to begin with all the attributes in a tuple and
>reduce to those attributes which uniquely identify the tuple. So
>everything inbetween, as well.

Let me add, if it helps to clarify anything:

Primary key - uniquely identified

	Superkey - uniquely identifies at a minimum
	Candidate - uniquely identifies
	Composite - but only if uniquely identifies
	Attribute - ""

Superkey - uniquely identifies, and perhaps then some
	Candidate - uniquely identifies
	Composite - but only if uniquely identifies
	Attribute - ""

Candidate Key - uniquely identifies
	Composite - but only if
	Attribute - ""

Composite Key - may uniquely identify, may not
	more than one Attribute	

Attribute - may uniquely identify or not
	one Attribute