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.

->where(['IS NOT', 'column', null]);

get

WHERE column IS NOT NULL

You can also use, it is faster to type

->where('column IS NOT NULL')

In complex query

->where(['AND',
  'column1 IS NOT NULL', // works
  ['IS NOT', 'column2', null], // works
  ['column3' => $value],
)

One of the options will be:

$query = new Query;             
$query->select('ID, City,State,StudentName')
    ->from('student')
    ->where(['IsActive' => 1])
    ->andWhere(['<>', 'City', null])
    ->andWhere(['<>', 'State', null])
    ->orderBy(['rand()' => SORT_DESC])
    ->limit(10);

Check official docs for where.