Example model

class Notification(TimeStampedModel):

    read_datetime = models.DateTimeField(
        '읽은 일시',
        null=True,
        default=None,
    )

filter(field__is_null=False)

In [2]: Notification.objects.filter(read_datetime__isnull=False)
Out[2]: SELECT "communications_notification"."id",
       "communications_notification"."read_datetime"
  FROM "communications_notification"
 WHERE "communications_notification"."read_datetime" IS NOT NULL
 LIMIT 21

Execution time: 0.004612s [Database: default]

exclude(field=None)

In [1]: Notification.objects.exclude(read_datetime=None)
Out[1]: SELECT "communications_notification"."id",
       "communications_notification"."read_datetime"
  FROM "communications_notification"
 WHERE NOT ("communications_notification"."read_datetime" IS NULL)
 LIMIT 21

Execution time: 0.007794s [Database: default]