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,

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.