A natural key (also known as business key or domain key) is a type of unique key in a database formed of attributes that exist and are used in the external world outside the database (i.e. in the business domain or domain of discourse). In the relational model of data, a natural key is a superkey and is therefore a functional determinant for all attributes in a relation.
A natural key serves two complementary purposes:
It provides a means of unique identification for data
It imposes a rule, specifically a uniqueness constraint, to ensure that data remains unique within an information system
The uniqueness constraint assures uniqueness of data within a certain technical context (e.g. a set of values in a table, file or relation variable) by rejecting input of any data that would otherwise violate the constraint. This means that the user can rely on a guaranteed correspondence between facts identified by key values recorded in a system and the external domain of discourse (a single version of the truth according to Kimball).
A natural key differs from a surrogate key which has no meaning outside the database itself and is not based on real-world observation or intended as a statement about the reality being modelled. A natural key therefore provides a certain data quality guarantee whereas a surrogate does not. It is common for elements of data to have several keys, any number of which may be natural or surrogate.
The advantages of using a natural key to uniquely identify records in a relation include less disk space usage, the natural key is an attribute that is related to the business or the real world so in most cases, it is already being stored in the relation which saves disk space as compared to creating a new column for storing the surrogate key.
Another advantage of using natural keys is that it simplifies enforcement of data quality, and they are easier to relate to real life while designing the database system. They simplify the quality of data as using a natural key that is unique in the real world ensures that there cannot be multiple records with the same primary key.
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In database design, a composite key is a candidate key that consists of two or more attributes (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. Composite keys have advantages similar to that of a natural key as it is often composed of multiple natural key attributes.
In the relational model of databases, a primary key is a specific choice of a minimal set of attributes (columns) that uniquely specify a tuple (row) in a relation (table). Informally, a primary key is "which attributes identify a record," and in simple cases constitute a single attribute: a unique ID. More formally, a primary key is a choice of candidate key (a minimal superkey); any other candidate key is an alternate key.
A relational database is a (most commonly digital) database based on the relational model of data, as proposed by E. F. Codd in 1970. A system used to maintain relational databases is a relational database management system (RDBMS). Many relational database systems are equipped with the option of using SQL (Structured Query Language) for querying and updating the database. The term "relational database" was first defined by E. F. Codd at IBM in 1970. Codd introduced the term in his research paper "A Relational Model of Data for Large Shared Data Banks".
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