如何使用 Hibernate 注释创建允许 NULL 值的唯一约束？

Question

I am using: Spring 4, Hibernate 4, SQL Server 2008

I know how to do it with SQL Server 2008 from this question response How do I create a unique constraint that also allows nulls?

But since I don't generate any manual SQL code during the creation of the table, is it possible to generate a "where clause" in my constraint through Hibernate annotations in my Entity class?

My DDL is created from scratch with the java entity definition as follows:

@Entity
@Table(name="Neighborhood", 
uniqueConstraints = {@UniqueConstraint(columnNames = {"codecnbv","zipcode"})})
@JsonSerialize(include = JsonSerialize.Inclusion.NON_EMPTY)
public class Neighborhood implements Serializable {

private String id;
private String codecnbv;
private String zipcode;

@Id 
@Column(name="id", nullable=false, unique=true, length=2)
public String getId() {
    return this.id;
}

@Column(name="codecnbv", nullable=true, length=12) //explicitly nullable
public String getCodecnbv() {
    return codecnbv;
}

@Column(name="zipcode", nullable=true, length=5) //explicitly nullable
public String getZipcode() {
    return zipcode;
}

}

However, as soon as I add data and try to enter a second record with NULL in column codecnbv and/or zipcode, I receive an exception that says I've violated the unique constraint.

The requirement I have says that I must allow multiple null values, and when the value is not null, then I should have unique values ie

For zipcode column

1. 56000 --ok
2. NULL --ok
3. 34089 --ok
4. NULL --ok
5. 34089 --Not allowed
6. 34567 --ok

Answer 1

It is not an issue of Hibernate but of SQL Server, which considers NULL a value and does not allow a second NULL value. Wicked, I know.

Some links:

How do I create a unique constraint that also allows nulls?

http://sqlmag.com/database-development/multiple-nulls-unique-constraints