Yii2怎么执行原生sql语句查询

先创建连接对象
$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();