octosql

OctoSQL is predominantly a CLI tool which lets you query a plethora of databases and file formats using SQL through a unified interface, even do JOINs between them. (Ever needed to join a JSON file with a PostgreSQL table? OctoSQL can help you with that.)

At the same time it's an easily extensible full-blown dataflow engine, and you can use it to add a SQL interface to your own applications.