code: public <T> T queryColumn(String sql, Object... paras) {
  		List<T> result = query(sql, paras);
  		if (result.size() > 0) {
  			T temp = result.get(0);
  			if (temp instanceof Object[])
  				throw new ActiveRecordException("Only ONE COLUMN can be queried.");
  			return temp;
  		}
  		return null;
  	}
docs: Execute sql query just return one column.
  @param <T> the type of the column that in your sql's select statement
  @param sql an SQL statement that may contain one or more '?' IN parameter placeholders
  @param paras the parameters of sql
  @return <T> T