What is unique key example?

What is meant by unique key?

In relational database management systems, a unique key is a candidate key that is not the primary key of the relation. 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.

Which key is a unique key?

The primary key is accepted as a unique or sole identifier for every record in the table. In the case of a primary key, we cannot save NULL values. In the case of a unique key, we can save a null value, however, only one NULL value is supported.

Is unique key a primary key?

A primary key should be unique, but a unique key cannot necessarily be the primary key. The primary key by default is a clustered index where data is physically organized in the sequential index. In contrast, the unique key is a unique non-clustered index.

How do I use a unique key?

Sometimes we want to add a unique key to the column of an existing table; then, this statement is used to add the unique key for that column. Following are the syntax of the ALTER TABLE statement to add a unique key: ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE(column_list);