Code:

select * from sessions ses
left join korisnici using (id_korisnika)
where session_id = 'neki-session-id';

Code:

    CachedRowSetImpl crs = null;
    try {

        sqlstr = "SELECT * FROM  sessions ses "
                + "LEFT JOIN korisnici kor USING (id_korisnika) "
                + "WHERE ses.session_id = '" + session_id + "' ";
        crs = db.executeQuery(sqlstr, 0);
        crs.last();
        int broj_slogova = crs.getRow();
        if (broj_slogova == 0) {
             return "BEZ_PRIJAVE|-";
        }

        str_check_time = crs.getString("check_time"); <<< zvekete!!

       .....
   } catch (SQLException ex) {
      System.out.println("[" + new java.util.Date() + "]\nSQLException: " + ex.getMessage()
   }

/////////////
/// class db
/////////////
public static CachedRowSetImpl executeQuery(String query, int tip) {
        Connection conn = null;
        Statement st = null;
        ResultSet rs = null;

        try
        {
            conn = getConnection();

            switch (tip)
            {
                case 0://SELECT
                    st = conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);
                    rs = st.executeQuery(query);

                    crs = new CachedRowSetImpl();
                    crs.populate(rs);

                    break;

                default://INSERT, UPDATE, or DELETE
                    st = conn.createStatement();
                    st.executeUpdate(query);
                    break;

            }
            
        } catch (SQLException ex)
        {
            System.out.println("[" + new java.util.Date() + "] GRESKA PRILIKOM IZVRSAVANJA SQL UPITA!\n" + query +"\nOPIS GRESKE:\n"+ ex);
        }

        try
        {
            if (st != null)
            {
                st.close();
            }
            if (rs != null)
            {
                rs.close();
            }

            if (conn != null)
            {
                freeConnection(conn);
            }
        } catch (Exception e)
        {
            e.printStackTrace();
        }


        return crs;

    }

    public static synchronized void freeConnection(Connection connection) {
        try
        {
            if (connection != null)
            {
                connection.close();
            }
            connection = null;
        } catch (Exception e)
        {
            System.err.println("DBBroker: Threw an exception closing a database connection");
            e.printStackTrace();
        }
    }