code: @Override
      protected StringBuilder getAlterColumnIsNotNull(final String _columnName,
                                                      final boolean _isNotNull)
      {
          final StringBuilder ret = new StringBuilder()
              .append(" alter column ").append(getColumnQuote()).append(_columnName).append(getColumnQuote());
          if (_isNotNull) {
              ret.append(" set ");
          } else {
              ret.append(" drop ");
          }
          ret.append(" not null");
          return ret;
      }
docs: {@inheritDoc}