docs: def get_update_sql(self, rows):
          """
          Returns SQL UPDATE for rows ``rows``
  
          .. code-block:: sql
  
              UPDATE table_name
              SET
                  field1 = new_values.field1
                  field2 = new_values.field2
              FROM (
                  VALUES
                      (1, 'value1', 'value2'),
                      (2, 'value1', 'value2')
              ) AS new_values (id, field1, field2)
              WHERE table_name.id = new_values.id;
  
          """
          field_names = self.get_field_names()
          pk = field_names[0]
          update_field_names = field_names[1:]
  
          num_columns = len(rows[0])
          if num_columns < 2:
              raise Exception('At least 2 fields must be passed to get_update_sql')
  
          all_null_indices = [
              all(row[index] is None for row in rows)
              for index in range(1, num_columns)
          ]
  
          field_names_sql = '({0})'.format(', '.join(field_names))
  
          row_values = []
          sql_args = []
  
          for row in rows:
              placeholders = []
              for value in row:
                  sql_args.append(value)
                  placeholders.append('%s')
              row_values.append('({0})'.format(', '.join(placeholders)))
          row_values_sql = ', '.join(row_values)
  
          # build field list for SET portion
          set_field_list = [
              '{0} = NULL'.format(field_name)
              if all_null_indices[idx] else '{0} = new_values.{0}'.format(field_name)
              for idx, field_name in enumerate(update_field_names)
          ]
          set_field_list_sql = ', '.join(set_field_list)
  
          self.sql = 'UPDATE {0} SET {1} FROM (VALUES {2}) AS new_values {3} WHERE {0}.{4} = new_values.{4}'.format(
              self.tables[0].get_identifier(),
              set_field_list_sql,
              row_values_sql,
              field_names_sql,
              pk
          )
  
          return self.sql, sql_args