Harlequin is a terminal-based SQL IDE that runs queries against DuckDB and SQLite databases, with a plug-in adapter system for connecting to additional database engines. It provides a full-screen text editor with syntax highlighting and fuzzy autocomplete for writing SQL, and displays query results in a scrollable table within the terminal.
The application distinguishes itself through a tree-based data catalog that lets you browse database schemas, local files, and remote S3 objects, with the ability to insert or copy paths directly into the query editor. It supports custom key bindings through installable keymap plugins or TOML configuration files, and offers visual theme selection and a full-screen mode toggle for focused work. Configuration is managed through multiple TOML files that merge with command-line flags, and named profiles can persist sets of startup options.
The tool includes terminal diagnostics for checking locale formatting, color display, clipboard operations, and key binding forwarding across platforms. It can be installed via Homebrew, Nix, pip, pipx, or uv, and provides a configuration wizard for generating TOML files interactively.