UNION query creates weird characters - sql

Question

In my Access application I join two resultsets with UNION. This worked for years, but now it stopped working, because after the union one text field just shows some crude chinese characters.
This is the setup:
1. select id,fieldA,1 as fieldB from tableA where cond=1
2. select id,"" as fieldA,2 as fieldB from tableA where cond=0
These queries seperately show the correct results. But after I join them with
select * from subquery1 UNION select * from subquery2
the data from fieldA is just some chinese characters like 㼄W. I have no idea where this comes from.
After some trying I found out that the following query shows the correct results:
select * from subquery1 where id=1 UNION select * from subquery2 where id=1
A nice side-effect is the performance improvement, although I have to change the querydef each time. But how come this works and the old version stopped working?

Answer 1

Instead of showing an empty string "", try adding Null
SELECT id, fieldA, 1 as fieldB FROM tableA WHERE cond=1
UNION SELECT id,Null as fieldA,2 as fieldB FROM tableA where cond=0;

Answer 2

For information, I solved a similar issue simply by using UNION ALL, with the side benefit of an improved performance.

Answer 3

The problem is that Access uses the types in the first SELECT statement in a union query to determine the types of the fields in the query. If you use a null value in this first select block, it has no idea what type to use and messes up. The simplest solution is to re-order the select blocks in your query so that the first one does not contain a null entry. Alternatively, the most generic solution is to create a null table with one field of each type you need, but no data in the table :-
Null Values Table
Field Name Data Type
Null Id AutoNumbered
Null Int Number
Null Currency Currency
Null Date Date/time
Null Text Short Text
Then incorporate a dummy SELECT block prior to the first real block that uses the entries from this Null table :-
SELECT
[Null Date] AS [Date],
[Null Id] AS [Index],
[Null Text] AS [Description],
[Null Currency] AS [Income Tax],
[Null Currency] AS [Employers NI],
[Null Currency] AS [Employees NI],
[Null Currency] AS [Amounts Due],
[Null Currency] AS [Payments],
[Null Currency] AS [Balance]
FROM [Null Values Table];
UNION SELECT
[Date],
Int(Null) AS [Index],
"Salary" AS [Description],
[Income Tax],
[Employers NI],
[Employees NI],
[Income Tax] + [Employers NI] + [Employees NI] AS [Amounts Due],
Int(Null) AS [Payments],
[Income Tax] + [Employers NI] + [Employees NI] AS [Balance]
FROM [Salaries Table];
UNION SELECT ...