Unique key

In database relational modeling and implementation, a unique key is a superkey—that is, in the relational model of database organization, a set of attributes of a relation variable for which it holds that in all relations assigned to that variable, there are no two distinct tuples (rows) that have the same values for the attributes in this set.

When more than one column is combined to form a unique key, their combined value is used to access each row and maintain uniqueness. These keys are referred to as aggregate or compound keys. Values are not combined, they are compared using their data types.

When a column or set of columns is defined as unique to the database management system, the system verifies that each set of values is unique before assigning the constraint. After the column(s) is(are) defined as unique, an error will occur if an insertion is attempted with values that already exist. Some systems will not allow key values to be updated, all systems will not allow duplicates. This ensures that uniqueness is maintained in both the primary table and any relations that are later bound to it.

Keys provide the means for database users and application software to identify, access and update information in a database table. There may be several keys in any given table. For example, two distinct keys in a table of employees might be employee number and login name. The enforcement of a key constraint (i.e. a uniqueness constraint) in a table is also a data integrity feature of the database. The DBMS prevents updates that would cause duplicate key values and thereby ensures that tables always comply with the desired rules for uniqueness. Proper selection of keys when designing a database is therefore an important aspect of database integrity.

A relational database table may have one or more available keys (formally called candidate keys). One of those keys per table may be designated the "primary" key, alternatively another key ("surrogate key") may be used. Any remaining keys are called alternate, or secondary, keys. Although mainly used today in the relational database context, the terms primary key and secondary key pre-date the relational model and are also used in other database models.

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