code: private static String joinAndGetValueForSelect(List<Field> fields, String sep, String fieldPrefix) {
          fieldPrefix = fieldPrefix == null ? "" : fieldPrefix.trim();
  
      	StringBuilder sb = new StringBuilder();
      	for(Field field : fields) {
      		Column column = field.getAnnotation(Column.class);
      		
      		String computed = column.computed().trim();
      		if(!computed.isEmpty()) {
      			sb.append("(").append(computed).append(") AS ");
      		} else {
      			sb.append(fieldPrefix); // 计算列不支持默认前缀，当join时，请自行区分计算字段的命名
      		}
          	sb.append(getColumnName(column)).append(sep);
      	}
      	int len = sb.length();
      	return len == 0 ? "" : sb.toString().substring(0, len - 1);
  	}
docs: 拼凑字段逗号,分隔子句（用于select）。会处理computed的@Column字段
  @param fields
  @param sep
  @param fieldPrefix
  @return