When having a JPA unique constraint on a field, such as:

@Table(name = "my_table", uniqueConstraints = {
    @UniqueConstraint(columnNames = {
        "my_field"
    })

Hiberbate generates the following SQL statement:

create unique index <generated_name> on my_table(my_field)

For MySQL/MariaDB and PostgreSQL, such an index allows multiple null values if my_field is nullable. However, for JavaDB, this fails. To allow multiple null values, JavaDB requires a statement like the following:

alter table my_table add constraint <generated_name> unique(my_field)

This statement will allow multiple null values.