@Entity
@Table(name = "Documents")
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name = "discriminator", discriminatorType = DiscriminatorType.STRING)
public abstract class Document implements Serializable
{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column
    protected Integer id;

    @Column(name = "file_name")
    protected String fileName;

    ...
}

@Entity
@Table(name = "ExternalDocuments")
@DiscriminatorValue(value = "external")
public class ExternalDocument extends SystemDocument
{
    ... // no additional fields
}