$connection = new \yii\db\Connection([    ‘dsn‘ => $dsn,     ‘username‘ => $username,     ‘password‘ => $password,]);$connection->open();

//查询返回多行：$command = $connection->createCommand(‘SELECT * FROM post‘);$posts = $command->queryAll();//返回单行：$command = $connection->createCommand(‘SELECT * FROM post WHERE id=1‘);$post = $command->queryOne();//查询多行单值：$command = $connection->createCommand(‘SELECT title FROM post‘);$titles = $command->queryColumn();//查询标量值/计算值：$command = $connection->createCommand(‘SELECT COUNT(*) FROM post‘);$postCount = $command->queryScalar();