'Given a lookup_type of \'year\', \'month\', \'day\', \'hour\', \'minute\', or
\'second\', return the SQL that extracts a value from the given
datetime field field_name.'

def datetime_extract_sql(self, lookup_type, field_name, tzname):

    raise NotImplementedError('subclasses    of    BaseDatabaseOperations    may    require    a    datetime_extract_sql()    method')
