langchain/chains/sql_database/base.py/SQLDatabaseChain/output_keys
class SQLDatabaseChain:    def output_keys(self) -> List[str]:
        """Return the singular output key.

        :meta private:
        """
        if not self.return_intermediate_steps:
            return [self.output_key]
        else:
            return [self.output_key, INTERMEDIATE_STEPS_KEY]