code: public <T> T queryFirst(String sql, Object... paras) {
  		List<T> result = query(sql, paras);
  		return (result.size() > 0 ? result.get(0) : null);
  	}
docs: Execute sql query and return the first result. I recommend add "limit 1" in your sql.
  @param sql an SQL statement that may contain one or more '?' IN parameter placeholders
  @param paras the parameters of sql
  @return Object[] if your sql has select more than one column,
  and it return Object if your sql has select only one column.