private Long id;
    private String fio;
    private SprGroup sprGroup;
    private SprTypeUch sprTypeUch;
    private List<Posesh> poseshList;

while (rs.next()) {
                std = new SprStudent(rs.getLong("id"), rs.getString("fio"), ... , ... , ...);
                listStd.add(std);
            }

....
String sql = "select id, fio, id_group from student";
        ConnDB connDB = new ConnDB();
        try {
            conn = connDB.getConn();
            ps = conn.prepareStatement(sql);
            rs = ps.executeQuery();
            while (rs.next()) {
                std = new Student(rs.getLong("id"), rs.getString("fio"),
                        rs.getObject("id_group", Group.class)); // именно в этой строке что должен быть? 
                listStd.add(std);
            }
....

SELECT 1,2,2 FROM Table1
JOIN Table2 on Table1.ID = Table1.ID

select o from Table o

public class Student {

    private Long id;
    private String fio;
    private int idGr;
    
    public SprStudent() {
    }

    public SprStudent(Long id, String fio, int idGr) {
        this.id = id;
        this.fio = fio;
        this.idGr=idGr;
    }

public class SprGroup implements Serializable{
    private Long id;
    private String name;

    public SprGroup() {
    }

    public SprGroup(Long id, String name) {
        this.id = id;
        this.name = name;
    }