Is composite key a subset of a pool of candidate keys? If it is a subset then why there is a new terminology for composite key, if candidate key can also formed with multiple columns.
Update
Simple Key - A simple key is one that has only one attribute.
Candidate Key - a candidate key of a relation is a minimal super key for that relation; that is, a set of attributes such that:
a. The relation does not have two distinct tuples (i.e. rows or records in common database language) with the same values for these attributes (which means that the set of attributes is a super key)
b. There is no proper subset of these attributes for which (1) holds (which means that the set is minimal).
I would like to insist the point that candidate keys are minimal super key.
Compound key - a compound key is a key that consists of two or more attributes that uniquely identify an entity occurrence.
- A composite key contains at least one compound key and one more attribute. Composite keys may also include simple keys and non-key attributes.
Now composite key does not satisfy the condition of candidate key of minimal super key.
I may be wrong but please help me in understanding this concept.
