private void clearReferences(DatabaseUpdater updater, MergeMapping mapping)
			throws UpdateException {

		if (mapping.isManyToMany()) {
			String query = "DELETE FROM " + DB.ESCAPE + ""
					+ mapping.getMappingTable() + "" + DB.ESCAPE + " WHERE "
					+ DB.ESCAPE + "" + mapping.getColumnFrom() + "" + DB.ESCAPE
					+ " = '" + mapping.getValueFrom() + "'";

			if (!updater.execute(query)) {
				throw new UpdateException(
						"unable to insert relation to mappingtable");
			}
		} else if (mapping.isManyToOne()) {
			String query = "UPDATE " + DB.ESCAPE + ""
					+ mapping.getMappingTable() + "" + DB.ESCAPE + " SET "
					+ DB.ESCAPE + "" + mapping.getColumnFrom() + "" + DB.ESCAPE
					+ " = 'NULL' WHERE " + DB.ESCAPE + ""
					+ mapping.getColumnFrom() + "" + DB.ESCAPE + " = '"
					+ mapping.getValueFrom() + "'";

			if (!updater.execute(query)) {
				throw new UpdateException(
						"unable to insert relation to mappingtable");
			}
		}
	}