code: @Override
      public QClassEqual appendSQL(final SQLSelect _sql)
          throws EFapsException
      {
          getAttribute().appendSQL(_sql);
          final Classification clazz = this.values.get(0).getClassification();
          final Type relType = clazz.getClassifyRelationType();
          _sql.addPart(SQLPart.IN).addPart(SQLPart.PARENTHESIS_OPEN).addPart(SQLPart.SELECT)
              .addColumnPart(null, relType.getAttribute(clazz.getRelLinkAttributeName()).getSqlColNames().get(0))
              .addPart(SQLPart.FROM)
              .addTablePart(relType.getMainTable().getSqlTable(), null)
              .addPart(SQLPart.WHERE)
              .addColumnPart(null, relType.getAttribute(clazz.getRelTypeAttributeName()).getSqlColNames().get(0));
  
          if (this.values.size() > 1) {
              _sql.addPart(SQLPart.IN).addPart(SQLPart.PARENTHESIS_OPEN);
              boolean first = true;
              for (final QClassValue value : this.values) {
                  if (first) {
                      first = false;
                  } else {
                      _sql.addPart(SQLPart.COMMA);
                  }
                  value.appendSQL(_sql);
              }
              _sql.addPart(SQLPart.PARENTHESIS_CLOSE);
          } else {
              _sql.addPart(SQLPart.EQUAL);
              getValue().appendSQL(_sql);
          }
          _sql.addPart(SQLPart.PARENTHESIS_CLOSE);
          return this;
      }
docs: {@inheritDoc}