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Concept
Unique key
Summary
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. Unique keys can consist of multiple columns. Unique keys are also called alternate keys. Unique keys are an alternative to the primary key of the relation. In SQL, the unique keys have a UNIQUE constraint assigned to them in order to prevent duplicates (a duplicate entry is not valid in a unique column). Alternate keys may be used like the primary key when doing a single-table select or when filtering in a where clause, but are not typically used to join multiple tables.
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, in a table of employees, both employee number and login name are individually unique. 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 unique keys (formally called candidate keys). One of those keys per table may be designated the primary key; other keys are called alternate keys.
Any key may consist of one or more attributes. For example, a Social Security Number might be a single attribute key for an employee; a combination of flight number and date might be a key consisting of two attributes for a scheduled flight.
There are several types of keys used in database modeling and implementations.
At the most basic definition, "a key is a unique identifier", so unique key is a pleonasm.
Official source
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