public static HashSet<ClassUserRelation> getClassUserRelationRows() {
        Connection conn = ConnectionManager.getInstance().getConnection();
        
        String sql = "{call selectClassUserRelationRows()}";

        try (PreparedStatement stmt = conn.prepareStatement(sql);
                ResultSet rs = stmt.executeQuery();) {

            HashSet<ClassUserRelation> hashSet = new HashSet<ClassUserRelation>();

            while (rs.next()) {
                ClassUserRelation classUserRelationEntity = new ClassUserRelation();
                classUserRelationEntity.setClassNo(rs.getString("classNo"));
                classUserRelationEntity.setUserNo(rs.getString("userNo"));

                hashSet.add(classUserRelationEntity);
            }
            return hashSet;
            
        } catch (SQLTransientConnectionException SQLtce) {
            return getClassUserRelationRows();
        } catch (SQLTransientException SQLte) {
            return getClassUserRelationRows();
        } catch (SQLException e) {
            System.err.println(e);
            return null;
        } finally {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }