Σε αυτήν τη σελίδα μπορείτε να λάβετε μια λεπτομερή ανάλυση μιας λέξης ή μιας φράσης, η οποία δημιουργήθηκε χρησιμοποιώντας το ChatGPT, την καλύτερη τεχνολογία τεχνητής νοημοσύνης μέχρι σήμερα:
In the relational data model a superkey is a set of attributes that uniquely identifies each tuple of a 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.
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.