Question:

When I use dbquery to find a field's null value, (e.g., dbquery(db,dbNULL,nullvalue);), it always returns "".

Answer:

The problem here is that dbquery(.., dbNULL, ..) returns a pointer, it doesn't copy the value into your string. The usage should be like this:

dbquery ( db, dbNULL, &null ) ;

You can also use the null record and dbgetv to get the value for a particular field, eg:

db.record = dbNULL ; 
dbgetv ( db, 0, "evid", &evid, 0 ) ;