code: def select_distinct_field(col, field_or_fields, filters=None):
      
      fields = _preprocess_field_or_fields(field_or_fields)
  
      if filters is None:
          filters = dict()
  
      if len(fields) == 1:
          key = fields[0]
          data = list(col.find(filters).distinct(key))
          return data
      else:
          pipeline = [
              {
                  "$match": filters
              },
              {
                  "$group": {
                      "_id": {key: "$" + key for key in fields},
                  },
              },
          ]
          data = list()
          for doc in col.aggregate(pipeline):
              # doc = {"_id": {"a": 0, "b": 0}} ...
              data.append([doc["_id"][key] for key in fields])
          return data
docs: Select distinct value or combination of values of
      single or multiple fields.
  
      :params fields: str or list of str.
      :return data: list of list.
  
      **中文文档**
  
      选择多列中出现过的所有可能的排列组合。