Uniqueness — is it something other than a key?

Uniqueness is a property of a dataset where the values of a specific field (or set of fields) do not repeat within that dataset. Unlike a key (for example, a primary key), uniqueness does not necessarily mean that this field identifies a record unambiguously or is used for linking with other tables.

For example, a database may have an email field that must be unique (the same email cannot appear for different users), but this is not necessarily a key if another attribute (such as id) is used for identification.

Thus, uniqueness is a constraint on the repetition of values, while a key is a concept of data identification and linkage.

Example:

• Key (primary key): user id — unique and unambiguously identifies a record.
• Uniqueness: user email — unique but not necessarily a key.

In Java, this is often implemented through annotations @UniqueConstraint or @Column(unique = true) in JPA, which differ from @Id, which defines a key.