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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def query(self, sql_query, return_as="dataframe"):
 """ Execute a raw SQL query against the the SQL DB. Args: sql_query (str):
 A raw SQL query to execute. Kwargs: return_as (str):
 Specify what type of object should be returned. The following are acceptable types: - "dataframe": pandas.DataFrame or None if no matching query - "result": sqlalchemy.engine.result.ResultProxy Returns: result (pandas.DataFrame or sqlalchemy ResultProxy):
 Query result as a DataFrame (default) or sqlalchemy result (specified with return_as="result") Raises: QueryDbError """

        if isinstance(sql_query, str):
            pass
        elif isinstance(sql_query, unicode):
            sql_query = str(sql_query)
        else:
            raise QueryDbError("query() requires a str or unicode input.")

        query = sqlalchemy.sql.text(sql_query)

        if return_as.upper() in ["DF", "DATAFRAME"]:
            return self._to_df(query, self._engine)
        elif return_as.upper() in ["RESULT", "RESULTPROXY"]:
            with self._engine.connect() as conn:
                result = conn.execute(query)
                return result
        else:
            raise QueryDbError("Other return types not implemented.")