How to put a unique database constrain correctly – using Alter Table or in JPA annotation?

I work with MySQL/OPENJPA. I have a task to impose uniqueness in one of the table columns. From what I see I can do this in 2 ways.

1. Use Alter Table ADD UNIQUE

OR

2. Using JPA annotation: @Table(name="", uniqueConstraints=@Unique(columnNames=""))

Which way is correct? Or maybe using both at the same time?