DuckDB

DuckDB is a high-performance, in-process SQL database for analytical workloads. It runs inside your application rather than as a separate server, and is designed to be fast, reliable, portable, and easy to use across many environments.

Its SQL dialect goes well beyond the basics, with arbitrary and nested correlated subqueries, window functions, collations, and complex types such as arrays, structs, and maps. CSV and Parquet files can be queried directly by referencing them in the FROM clause, so file analysis stays inside SQL.

DuckDB ships as a standalone CLI and as clients for Python, R, Java, and Wasm, with deep integrations into tools like pandas and dplyr. It is open source under the MIT license.