Small Swoole Db 2.3 で左結合が導入されました :

$selector = (new TableSelector('user'))
    ->leftJoin('post', 'messageOwner', 'message')
;

$selector->where()
    ->firstCondition(new Condition(
        new ConditionElement(ConditionElementType::var, 'name', 'user'),
        ConditionOperator::equal,
        new ConditionElement(ConditionElementType::const, 'john')
    ))->andCondition(new Condition(
        new ConditionElement(ConditionElementType::var, 'subject', 'message'),
        ConditionOperator::like,
        new ConditionElement(ConditionElementType::const, '%hiring%')
    ))
;

packagist : https://packagist.org/packages/small/swoole-db
git : https://git.small-project.dev/lib/small-swoole-db