code: @Override
      public int join(final OneSelect _oneSelect,
                      final SQLSelect _select,
                      final int _relIndex)
          throws EFapsException
      {
          Integer ret;
          final String tableName = this.classification.getMainTable().getSqlTable();
          final Attribute attr = this.classification.getAttribute(this.classification.getLinkAttributeName());
          if (attr == null) {
              ClassSelectPart.LOG.error("Could not find attribute: '{}'", this.classification.getLinkAttributeName());
              throw new EFapsException(ClassSelectPart.class, "joinNoAttribute");
          }
          final String column = attr.getSqlColNames().get(0);
          ret = _oneSelect.getTableIndex(tableName, column, _relIndex, this.classification.getId());
          if (ret == null) {
              ret = _oneSelect.getNewTableIndex(tableName, column, _relIndex, this.classification.getId());
              _select.leftJoin(tableName, ret, column, _relIndex, "ID");
          }
          this.tableIdx = ret;
          return ret;
      }
docs: {@inheritDoc}