code: public static <T> T queryColumn(String sql, Object... paras) {
  		return MAIN.queryColumn(sql, paras);
  	}
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