You can't in Access. The absolute (no you can't change it) limit is 255 fields
in a query and 2000 characters (not including memo fields).
One possible workaround would be to use some vba to combine the data from
multiple columns into one memo column or even one text column. That would
require you passing the primary key or keys of the relavant record and then
extracting the relevant data and combining them in the vba function.
Perhaps, you can explain why you need 400 fields/columns in your query. With
luck, someone may have an alternative solution for you - perhaps, involving
sub-forms or sub-reports.
