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The columns in a candidate key are called prime attributes, [3] and a column that does not occur in any candidate key is called a non-prime attribute. Every relation without NULL values will have at least one candidate key: Since there cannot be duplicate rows, the set of all columns is a superkey, and if that isn't minimal, some subset of that ...
There is a multi-attribute unique identifier/candidate key: "Manufacturer" and "Model". {Manufacturer country} is functionally dependent (predictable) on {Manufacturer}. {Manufacturer} is a proper subset of the {Manufacturer, Model} candidate key. {Manufacturer country} is not part of a candidate key, so it is a non-prime attribute.
In relational database management systems, a unique key is a candidate key. All the candidate keys of a relation can uniquely identify the records of the relation, but only one of them is used as the primary key of the relation. The remaining candidate keys are called unique keys because they can uniquely identify a record in a relation.
A candidate key (or minimal superkey) is a superkey that can't be reduced to a simpler superkey by removing an attribute. [ 3 ] 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 ...
Usually one candidate key is chosen to be called the primary key and used in preference over the other candidate keys, which are then called alternate keys. A candidate key is a unique identifier enforcing that no tuple will be duplicated; this would make the relation into something else, namely a bag, by violating the basic definition of a set ...
For referential integrity to hold in a relational database, any column in a base table that is declared a foreign key can only contain either null values or values from a parent table's primary key or a candidate key. [2] In other words, when a foreign key value is used it must reference a valid, existing primary key in the parent table.
To conform to 2NF and remove duplicates, every non-candidate-key attribute must depend on the whole candidate key, not just part of it. To normalize this table, make {Title} a (simple) candidate key (the primary key) so that every non-candidate-key attribute depends on the whole candidate key, and remove Price into a separate table so that its ...
A table is in 4NF if and only if, for every one of its non-trivial multivalued dependencies X Y, {X, Y} is a superkey—that is, the combination of all attributes in X and Y is either a candidate key or a superset thereof. [1]