candidate key

It follows that every relation will have at least one candidate key.

The previous example only illustrates the definition of a candidate key and not how these are determined in practice.

The definition of candidate keys can be illustrated with the following (abstract) example.

The candidate keys of a relation tell us all the possible ways we can identify its tuples.

Where such sets are themselves a candidate key, the index can be a unique index.

These are indeed the candidate keys of relvar R.

Each such set constitutes a candidate key.

Candidate key - minimal superkey for a relation.

These are known as candidate keys.

From the set of candidate keys, a single unique key is selected and declared the primary key for that data entity.

Multiple candidate keys occur in the following table:

Foreign keys are integrity constraints enforcing that the value of the attribute set is drawn from a candidate key in another relation.

There were in fact a number of possible candidate keys, for example, 'module-name' and 'course-name', but we chose the primary key as above.

The candidate keys of the table are:

In other words, a natural key is a candidate key that has a logical relationship to the attributes within that row.

We have to consider all the relations that might be assigned to a relvar to determine whether a certain set of attributes is a candidate key.

The bold, underlined attributes are candidate keys.

So, employeeID is a candidate key.

Because all three attributes are prime attributes (i.e. belong to candidate keys), the table is in 3NF.

In other situations there may be more than one candidate key for a relation, and no candidate key is obviously preferred.

In practice, several useful shorthands are expected to be available, of which the most important are candidate key (really, superkey) and foreign key constraints.

BCNF requires that all attribute values are fully dependent on each candidate key and not only the primary key.

Conversely, an attribute that does not occur in ANY candidate key is called a non-prime attribute.

A table is said to be in the 5NF if and only if every join dependency in it is implied by the candidate keys.

A candidate key is a minimal set of attributes necessary to identify a tuple, this is also called a minimal superkey.