What is a natural identifier in Hibernate?

Question

While reading through the Hibernate documentation, I keep seeing references to the concept of a natural identifier.

Does this just mean the id an entity has due to the nature of the data it holds?

E.g. A user's name + password + age + something are used as a compound identitifier?

Answer 1

In relational database theory a relation can have multiple candidate keys. A candidate key is a set of attributes of a relation that are never duplicate in two rows of that relation and that cannot be reduced by removing one of the attributes and still guarantee uniqueness.

A natural ID is essentially a candidate key. Where "natural" means it is in the nature of the data you hold in that relation, not something you add like an autogenerated key. A natural ID can be composed of a single attribute. In general any attribute of a relation that is unique and not-null is a candidate key, and can be considered a natural id.

In Hibernate this annotation can be used simply to denote that an attribute can be used to do searches that return unique results while not using the key. This can be useful when the attribute you denote as natural id is more natural to deal with for you, e.g. when the actual key is autogenerated and you don't want to use in searches.