select tmp from tbl  
(select * from db_emails where folder="spam") tmp
where tmp.digest=md5(concat("digest",?))

@Entity
@Table(name = "db_emails")
class Email implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id")
    private Long id;
    @Column(name = "title")    
    private String title;
    @Column(name = "from")    
    private String from;
    @Column(name = "to")
    private String to;
    @Column(name = "content")    
    private String content;
    @Column(name = "digest")
    private String digest;
/*
Getters and setters are being declared here

*/

}

select tmp from db_emails  
(select * from db_emails where folder="spam") tmp
where tmp.digest=md5(concat("digest",?))