We have a Relational API in addition to SQL! Here are some examples for the Python Relational API client:

https://github.com/duckdb/duckdb/blob/master/examples/python...

Plus, if you are working in Python, you can use DuckDB as the engine underneath Ibis, Fugue, Siuba, or anything that works with SQLAlchemy (using the DuckDB-engine driver)! In R, you can use dplyr or dbplyr.

DuckDB's file format is one way to persist data (it uses a single file), but you can also write out to Parquet, or write out to Apache Arrow and then parquet (in a partitioned format I believe).

Disclaimer - I write docs for DuckDB!