how to use not null condition in YII2

You can use the not operator combined with the fields that should not be null to generate a IS NOT NULL SQL statement. Like this:

$query = new Query;             
$query->select('ID, City,State,StudentName')
      ->from('student')                               
      ->where(['IsActive' => 1])
      ->andWhere(['not', ['City' => null]])
      ->andWhere(['not', ['State' => null]])
      ->orderBy(['rand()' => SORT_DESC])
      ->limit(10);

Also check the examples in the documentation.