private static String join(List<Field> fields, String sep, String fieldPrefix) {
    	return joinAndGetValueForSelect(fields, sep, fieldPrefix);
    }
private static String join ( List < Field > fields , String sep , String fieldPrefix ) { return joinAndGetValueForSelect ( fields , sep , fieldPrefix ) ; }
拼凑select的field的语句
@param fields
@param sep
@param fieldPrefix
@return