The simplest idea I have is that you simply subtract one code from the
other. If the difference does not equal zero then exclude the record.

Is "supplier one" the same in both tables? If so why not have one table for
"supplier" and one table for "code" so you have a 1-to-many relationship?

I think you may have created some difficulty for yourself with the design.

Your last sentence describes a query that includes both tables.  In query
design view, right click the join line, click Join Properties, and choose
the appropriate join type.

However, I don't think that is quite what you need.  I agree that your
structure may be part of the problem.  Supplier should be stored in one
table.  Codes associated with that supplier should be stored in a related
table.  That way you could have any number of codes, and retrieving the list
would be a simple matter.

Having said that, if the only two fields you need are Supplier and Code you
could use SELECT DISTINCTROW instead of SELECT in the query's SQL.  In query
design view, click View > Properties, and set Unique Records to Yes.

The following works when there is only one record with a given Supplier and
Code in each table... as would be the case if the combination of Supplier
and Code were defined at the Primary Key, or as a Unique Index.  Other
configurations may require a different approach.

In the Query Builder add TABLE A and TABLE B to a new Query.  Drag and drop
to join the SUPPLIER field, and do the same for the CODE. Click each of the
join lines, choose Join Properties, and choose "All records from Table A and
only those that match from Table B".  In the query grid, drag down the
SUPPLIER and CODE fields from each Table.  In the first criteria line under
the SUPPLIER and CODE from Table B, enter Is Null (criteria on the same line
are AND conditions). And, if you don't want the empty Fields to display on
the Query, uncheck the Show box on each of these two Fields, also.

Larry Linson
Microsoft Access MVP