This simple plugin demonstrates a problem I'm having creating a sub-command * of `wp db`.

db-sub-command-test.php

<?php

/**
 * Plugin Name: DB Sub-Comannd Test
 * Description: Demonstrate problem creating a sub-command of `wp db`.
 * Version: 0.1.0
 * Author: Paul V. Biron/Sparrow Hawk Computing
 * Author URI: http://sparrowhawkcomputing.com/
 *
 * This simple plugin demonstrates a problem I'm having creating a sub-command
 * of `wp db`.  See https://wordpress.slack.com/archives/C02RP4T41/p1643579809090159.
 */

namespace SHC\DB_SUBCOMMAND_TEST;

use WP_CLI;
use WP_CLI_Command;

defined( 'ABSPATH' ) || die;

add_action(
	'cli_init',
	function() {
		WP_CLI::add_command( 'db connection', __NAMESPACE__ . '\\Connection_Command' );
	}
);

if ( class_exists( 'WP_CLI_Command' ) ) {
	/**
	 * Display info about the DB connection.
	 */
	class Connection_Command extends WP_CLI_Command {
		/**
		 * Display the DB connection status.
		 */
		public function status() {
			WP_CLI::line( 'this is a test' );
		}
	}
}