yii2原生sql

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();

要养成先查文档的习惯：
http://www.yiichina.com/doc/guide/2.0/db-dao