code: def query_feature_values(
              self,
              column,
              feature,
              distinct=True,
              contig=None,
              strand=None):
          
          query = """
              SELECT %s%s
              FROM %s
              WHERE 1=1
          """ % ("DISTINCT " if distinct else "", column, feature)
          query_params = []
  
          if contig:
              contig = normalize_chromosome(contig)
              query += " AND seqname = ?"
              query_params.append(contig)
  
          if strand:
              strand = normalize_strand(strand)
              query += " AND strand = ?"
              query_params.append(strand)
  
          rows = self.run_sql_query(query, query_params=query_params)
          return [row[0] for row in rows if row is not None]
docs: Run a SQL query against the sqlite3 database, filtered
          only on the feature type.