@MappedSuperclass
public class BaseEntity{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
}

@MappedSuperclass
public class User extends BaseEntity{

    @Column(name="first_name")
    private String firstName;

    @Column(name="last_name")
    private String lastName;
}

@Entity
@Table(name="users")
public class Customer extends User {

    @OneToMany(mappedBy = "customer")
    private List<Appointment> appointments;
}


Entity
@Table(name="users")
public class Provider extends User {

    @OneToMany(mappedBy = "provider")
    private List<Appointment> appointments;

    @ManyToMany
    @JoinTable(name="work_provider", joinColumns=@JoinColumn(name="provider_id"), inverseJoinColumns=@JoinColumn(name="work_id"))
    private List<Work> works;

}