Multiple database connections and Yii 2.0

First you need to configure your databases like below:

return [
'components' => [
    'db1' => [
        'class' => 'yii\db\Connection',
        'dsn' => 'mysql:host=localhost;dbname=db1name', //maybe other dbms such as psql,...
        'username' => 'db1username',
        'password' => 'db1password',
    ],
    'db2' => [
        'class' => 'yii\db\Connection',
        'dsn' => 'mysql:host=localhost;dbname=db2name', // Maybe other DBMS such as psql (PostgreSQL),...
        'username' => 'db2username',
        'password' => 'db2password',
    ],
],
];

Then you can simply:

// To get from db1
Yii::$app->db1->createCommand((new \yii\db\Query)->select('*')->from('tbl_name'))->queryAll()

// To get from db2
Yii::$app->db2->createCommand((new \yii\db\Query)->select('*')->from('tbl_name'))->queryAll()

If you are using an active record model, in your model you can define:

public static function getDb() {
    return Yii::$app->db1;
}

//Or db2
public static function getDb() {
    return Yii::$app->db2;
}

Then:

If you have set db1 in the getDb() method, the result will be fetched from db1 and so on.

ModelName::find()->select('*')->all();