// find the customers whose primary key value is 10
$customers = Customer::findAll(10);
$customer = Customer::findOne(10);
// the above code is equivalent to:
$customers = Customer::find()->where(['id' => 10])->all();
// find the customers whose primary key value is 10, 11 or 12.
$customers = Customer::findAll([10, 11, 12]);
$customers = Customer::find()->where(['IN','id',[10,11,12]])->all();
// the above code is equivalent to:
$customers = Customer::find()->where(['id' => [10, 11, 12]])->all();
// find customers whose age is 30 and whose status is 1
$customers = Customer::findAll(['age' => 30, 'status' => 1]);
// the above code is equivalent to:
$customers = Customer::find()->where(['age' => 30, 'status' => 1])->all();
// use params binding
$customers = Customer::find()->where('age=:age AND status=:status')->addParams([':age'=>30,':status'=>1])->all();
// use index
$customers = Customer::find()->indexBy('id')->where(['age' => 30, 'status' => 1])->all();
// get customers count
$count = Customer::find()->where(['age' => 30, 'status' => 1])->count();
// add addition condition
$customers = Customer::find()->where(['age' => 30, 'status' => 1])->andWhere('score > 100')->orderBy('id DESC')->offset(5)->limit(10)->all();
// find by sql
$customers = Customer::findBySql('SELECT * FROM customer WHERE age=30 AND status=1 AND score>100 ORDER BY id DESC LIMIT 5,10')->all();

// update status for customer-10
$customer = Customer::findOne(10);
$customer->status = 1;
$customer->update();
// the above code is equivalent to:
Customer::updateAll(['status' => 1], 'id = :id',[':id'=>10]);

// delete customer-10
Customer::findOne(10)->delete();
// the above code is equivalent to:
Customer::deleteAll(['status' => 1], 'id = :id',[':id'=>10]);

$subQuery = (new Query())->select('COUNT(*)')->from('customer');
// SELECT `id`, (SELECT COUNT(*) FROM `customer`) AS `count` FROM `customer`
$query = (new Query())->select(['id', 'count' => $subQuery])->from('customer');

// select
$customers = Yii::$app->db->createCommand('SELECT * FROM customer')->queryAll();
// update
Yii::$app->db->createCommand()->update('customer',['status'=>1],'id=10')->execute();
// delete
Yii::$app->db->createCommand()->delete('customer','id=10')->execute();
//transaction
// outer
$transaction1 = $connection->beginTransaction();
try {
  $connection->createCommand($sql1)->execute();
  // internal
  $transaction2 = $connection->beginTransaction();
  try {
    $connection->createCommand($sql2)->execute();
    $transaction2->commit();
  } catch (Exception $e) {
    $transaction2->rollBack();
  }
  $transaction1->commit();
} catch (Exception $e) {
  $transaction1->rollBack();
}

[
  'class' => 'yii\db\Connection',
  // master
  'dsn' => 'dsn for master server',
  'username' => 'master',
  'password' => '',
  // slaves
  'slaveConfig' => [
    'username' => 'slave',
    'password' => '',
    'attributes' => [
      // use a smaller connection timeout
      PDO::ATTR_TIMEOUT => 10,
    ],
  ],
  'slaves' => [
    ['dsn' => 'dsn for slave server 1'],
    ['dsn' => 'dsn for slave server 2'],
    ['dsn' => 'dsn for slave server 3'],
    ['dsn' => 'dsn for slave server 4'],
  ],
]