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PostgreSQL subqueries explained with simple examples

PostgreSQL subqueries allow one SQL query to run inside another, making it possible to solve filtering, aggregation, ...
Linux Journal

MariaDB/MySQL, PostgreSQL and SQLite3 - Comparing Command-Line Interfaces

Don't be afraid of using your chosen database's command-line client. I might as well say this up front: I don't like using GUI (aka non-command-line or graphical) tools with my databases. This is ...