Superkey

{{Short description|Set of attributes that uniquely identifies each tuple of a relation}}

{{about|the database design term|the keyboard button|Super key (keyboard button)}}

In the relational data model a superkey is any set of attributes that uniquely identifies each tuple of a relation.{{cite web |url=https://www.dcs.warwick.ac.uk/~hugh/TTM/CJD-on-EFC's-First-Two-Papers.pdf |title=Codd's First Relational Papers: A Critical Analysis |last=Date |first=Christopher |date=2015 |website=warwick.ac.uk |access-date=2020-01-04 |quote=Note that the extract allows a “relation” to have any number of primary keys, and moreover that such keys are allowed to be “redundant” (better: reducible). In other words, what the paper calls a primary key is what later (and better) became known as a superkey, and what the paper calls a nonredundant (better: irreducible) primary key is what later became known as a candidate key or (better) just a "key".}}

{{cite book|title=Introduction to Database Management Systems|publisher=Tata McGraw-Hill|page=77|isbn=9780070591196|year=2005|quote=no two tuples in any legal relation

}} Because superkey values are unique, tuples with the same superkey value must also have the same non-key attribute values. That is, non-key attributes are functionally dependent on the superkey.

The set of all attributes is always a superkey (the trivial superkey). Tuples in a relation are by definition unique, with duplicates removed after each operation, so the set of all attributes is always uniquely valued for every tuple. A candidate key (or minimal superkey) is a superkey that can't be reduced to a simpler superkey by removing an attribute.{{Cite journal|last=Saiedian|first=H.|date=1996-02-01|title=An Efficient Algorithm to Compute the Candidate Keys of a Relational Database Schema|url=https://academic.oup.com/comjnl/article-lookup/doi/10.1093/comjnl/39.2.124|journal=The Computer Journal|language=en|volume=39|issue=2|pages=124–132|doi=10.1093/comjnl/39.2.124|issn=0010-4620}}

For example, in an employee schema with attributes employeeID, name, job, and departmentID, if employeeID values are unique then employeeID combined with any or all of the other attributes can uniquely identify tuples in the table. Each combination, {employeeID}, {employeeID, name}, {employeeID, name, job}, and so on is a superkey. {employeeID} is a candidate key, since no subset of its attributes is also a superkey. {employeeID, name, job, departmentID} is the trivial superkey.

If attribute set K is a superkey of relation R, then at all times it is the case that the projection of R over K has the same cardinality as R itself.