/**
  * Add a "WHERE" clause to the given query.
  *
  * @param  \Nova\Database\Query\Builder  $query
  * @param  string  $key
  * @param  string  $extraValue
  * @return void
  */
 protected function addWhere(QueryBuilder $query, $key, $extraValue)
 {
     if ($extraValue === 'NULL') {
         $query->whereNull($key);
     } elseif ($extraValue === 'NOT_NULL') {
         $query->whereNotNull($key);
     } else {
         $query->where($key, $extraValue);
     }
 }