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When the data model is instantiated into a physical database, it is the key that the system uses the most when accessing the table, or joining the tables together when selecting data. Alternate A non-primary key that can be used to identify only one row in a table. Alternate keys may be used like a primary key in a single-table select. Foreign
A candidate key is a minimal superkey, [1] i.e., a superkey that doesn't contain a smaller one. Therefore, a relation can have multiple candidate keys, each with a different number of attributes. [2] Specific candidate keys are sometimes called primary keys, secondary keys or alternate keys.
In database design, a composite key is a candidate key that consists of two or more attributes, [1] [2] [3] (table columns) that together uniquely identify an entity occurrence (table row). A compound key is a composite key for which each attribute that makes up the key is a foreign key in its own right.
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 ...
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.
In the relational model of databases, a primary key is a designated attribute that can reliably identify and distinguish between each individual record in a table.The database creator can choose an existing unique attribute or combination of attributes from the table (a natural key) to act as its primary key, or create a new attribute containing a unique ID that exists solely for this purpose ...
In a database driven data dictionary, the fully qualifithcfted data element name may become the primary key, or an alternate key, of a Data Elements table of the data dictionary. The data element name typically conforms to ISO/IEC 11179 metadata registry naming conventions and has at least three parts: Object, Property and Representation term.
An example of a database that has not enforced referential integrity. In this example, there is a foreign key (artist_id) value in the album table that references a non-existent artist — in other words there is a foreign key value with no corresponding primary key value in the referenced table.