As I alluded to in an earlier post, I had troubles using the nz() function from a TADODataset in Delphi. Take the following SQL, which works just fine in Access itself:

SELECT DISTINCT ContestId, Hole, nz(Score,'MISSING')
FROM tblContestDetails

This query behaves like isnull() in MSSQL where if the Score column is null, it returns the value 'MISSING' instead. If Score isn't null, then the value of Score would be returned. To me, this syntax is very intuitive, corresponds well with other DBMS' isnull() functions, and clearly captures the intent of what you're trying to do in once concise statement. However, if you put this query in a TADODataset and try to open that dataset, you will be greeted with the following error: "Undefined function 'nz' in expression.". If you need this type of substitution in Access when executing from Delphi, I found the simplest way to get around this is to use the iif() and isnull() functions. It's more verbose, and I don't like it as much, but when you need things to work, some times you have to live with things that aren't aesthetically pleasing. The SQL above translates to this:

SELECT DISTINCT ContestId, Hole, iif(isnull(Score),'MISSING', Score)
FROM tblContestDetails