What does it mean for a key to be unique?

Question

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 key that has migrated to another entity. At the most basic definition, "a key is a unique identifier", so unique key is a pleonasm.

What's the difference between a primary key and a unique key?

A primary key can constitute one or more fields of a table to identify records in a table uniquely. On the other hand, a unique key prevents two rows from having duplicate entries in a column. A table cannot have more than one primary key in a relational database, while there can be multiple unique keys per table.

Should keys be unique?

A primary key should be unique, but a unique key cannot necessarily be the primary key. Primary key implements entity integrity, whereas the unique key enforces unique data. In the Primary Key, the default index is clustered, whereas in the unique key the default index is not clustered.

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.

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