code: def column_values_at_locus(
              self,
              column_name,
              feature,
              contig,
              position,
              end=None,
              strand=None,
              distinct=False,
              sorted=False):
          
  
          # TODO: combine with the query method, since they overlap
          # significantly
          require_string(column_name, "column_name", nonempty=True)
  
          contig = normalize_chromosome(contig)
  
          require_integer(position, "position")
  
          if end is None:
              end = position
  
          require_integer(end, "end")
  
          if not self.column_exists(feature, column_name):
              raise ValueError("Table %s doesn't have column %s" % (
                  feature, column_name,))
  
          if distinct:
              distinct_string = "DISTINCT "
          else:
              distinct_string = ""
  
          query = """
              SELECT %s%s
              FROM %s
              WHERE seqname = ?
              AND start <= ?
              AND end >= ?
  
          """ % (distinct_string, column_name, feature)
  
          query_params = [contig, end, position]
  
          if strand:
              query += " AND strand = ?"
              query_params.append(strand)
  
          tuples = self.connection.execute(query, query_params).fetchall()
  
          # each result is a tuple, so pull out its first element
          results = [t[0] for t in tuples if t[0] is not None]
  
          if sorted:
              results.sort()
          return results
docs: Get the non-null values of a column from the database
          at a particular range of loci