Nz IIf Standard SQL, is there such a thing?

I have written a database as part of a project and it works fine so far. It's in Access 2000 and I need to make it web based and so have opted for Java for various reasons. I know I can link to Access (or any database) through Java, but I would like users (this will be freeware) to be able to use whatever database they wish. Also Access seems pretty limited.

I have a form where by options are selected from previously entered data, i.e. picked from a list, using a combo box and the ID no's they pick up are picked up by the following SQL

WHERE (((Job.ObjectID)=Nz([Form]![JoBQueryID]![Object], [ObjectID]))

Data from Form/Combo Box
Data from Query Field

(There is more code, but you get the idea)

I have also tried =(IIf(IsNull([Forms], but this requires the([Forms]! after the ObjectID (Query field) again!

Can anyone advise some simple SQL that will work with most databases?

Also, creating average time, cost, etc. fields in an SQL Query? Any tips?

Thanks

NB - I am a newbie, allthough I've been battling with this for around two years on and off now!

I can upload a copy of a database I am working on for analysis if required.