JPA - @Column (unique=true) - What is really point of having 'unique' attribute?

Question

Suppose I am having 'subject' table

CREATE TABLE subject (id int PRIMARY KEY, name VARCHAR(255) **UNIQUE**) 

and associated Mapped Object,

@Entity @Table(name="subject") public class SubjectDO {     @Id     @Column(name="id")     int id;      @Column(name="name", unique=true)     String name;     ...     // Getter-Setter methods } 

When I try to save object having duplicate 'name' with and without 'unique=true' defined, I am getting similar behavior (same exception.) And it is obvious that JPA implementation can't really do anything unless reaching out to DB for checking.

What is the real use case for it?

(I am assuming here, unique constraint is defined at Database level too.)

Answer 1

unique in @Column is used only if you let your JPA provider create the database for you - it will create the unique constraint on the specified column. But if you already have the database, or you alter it once created, then unique doesn't have any effect.

Answer 2

unique=true in @Column annotation will be used only in DDL generation, it doesn't have any impact during runtime. The actual uniqueness checks happens in the database.