search criteria

Hi

I have a table with PgID, Under_PgID, PgType etc fields. The PgID is the primary key, autonumber. The Under_PgID contains the PgID number of a page that it is 'under' in the menu structure.

I have a search form that works quite well (bar one issue that is on another post) - but I want to be able to search for pages PgType = 1 AND Under_PgID IS NULL

I read in the forums that searching for null values isn't possible with DA, so I wondered if there is a way to create a temporary field that inserts a value where the above criteria are met. This would then enable me to put another search value in place.

For example, if PgType=5 AND Under_PgID IS NULL, insert 'top' into temporary field 'TopPage'.

Whichever solution is possible, I'd be obliged for guidance

Thanks
Chris