Access query inconsistently treats empty string as null - sql

Question

I have an application that is grabbing data from an Access database. I am seeking the minimum value of a column and the results I am getting back are inconsistent.
Have I run into a feature where Access inconsistently treating an empty string as a null depending on whether I add a filter or not, or is there something wrong with the way I am querying the data?
The column contains one blank value (not null) and several non-blank values that are all identical (about 30 instances of 'QLD'). The query I am using has a filter that involves multiple other tables, so that only the blank value and about half of the 'QLD' values are eligible.
It's probably easier to show the code and the effects rather than describe it. I have created a series of unioned queries which 'should' bring back identical results but do not.
Query:
SELECT 'min(LOC_STATE)' as Category
, min(LOC_STATE) as Result
FROM pay_run, pay_run_employee, employee, department, location
WHERE pr_id = pre_prid
AND em_location = loc_id
AND pre_empnum = em_empnum
AND em_department = dm_id
AND pr_date >= #2/24/2015#
AND pr_date <= #2/24/2016#
UNION ALL
(SELECT TOP 1 'top 1 LOC_STATE'
, LOC_STATE
FROM pay_run, pay_run_employee, employee, department, location
WHERE pr_id = pre_prid
AND em_location = loc_id
AND pre_empnum = em_empnum
AND em_department = dm_id
AND pr_date >= #2/24/2015#
AND pr_date <= #2/24/2016#
ORDER BY LOC_STATE)
UNION ALL
SELECT 'min unfiltered', min(loc_state)
FROM location
UNION ALL
(SELECT TOP 1 'iif is null', iif(loc_state is null, 'a', loc_state)
FROM location
ORDER BY loc_state)
Results:
Category Result
min(LOC_STATE) 'QLD'
top 1 LOC_STATE ''
min unfiltered ''
iif is null ''
If I do a minimum with the filter it brings back 'QLD' and not the empty string. At this stage it is possible that the empty string is not being included because it is treated as a null or the filter removes it.
The second query, which brings back the top 1 state using the filter shows that the empty string is not filtered out, which means that the Min function is ignoring the empty string.
The third query, which gets the minimum of the unfiltered table, brings back the empty string - so the minimum function does not exclude empty strings / treat them as null.
The fourth query, ensures that there is not a null in the empty string position.
My conclusion is that perhaps the inclusion of other tables and filter criteria is causing the empty string value to be treated as a null, but I feel that I must be missing something.
NB: I have a very similar query (date literals altered) that executes against the same data imported into a SQL Server database. It is correctly returning '' for all 4 queries.
Does anyone know why the empty string is ignored by the Min function in the first query?
PS: for those who prefer a query with joins
SELECT 'min(LOC_STATE)' as Category
, min(LOC_STATE) as Result
FROM (((pay_run
INNER JOIN pay_run_employee ON pay_run.pr_id = pay_run_employee.pre_prid)
INNER JOIN employee ON pay_run_employee.pre_empnum = employee.em_empnum)
INNER JOIN department ON employee.em_department = department.dm_id)
INNER JOIN location on employee.em_location = location.loc_id
WHERE
PR_DATE >= #2/24/2015# and
PR_DATE <= #2/24/2016#
union all
(SELECT TOP 1 'TOP 1 LOC_STATE'
, LOC_STATE
FROM (((pay_run
INNER JOIN pay_run_employee ON pay_run.pr_id = pay_run_employee.pre_prid)
INNER JOIN employee ON pay_run_employee.pre_empnum = employee.em_empnum)
INNER JOIN department ON employee.em_department = department.dm_id)
INNER JOIN location on employee.em_location = location.loc_id
WHERE
PR_DATE >= #2/24/2015# and
PR_DATE <= #2/24/2016#
order by LOC_STATE)
union all
select 'min unfiltered', min(loc_state)
from location

Answer 1

This has got nothing to do with corrupt data or unions or joins. The problem can be easily made visible by exectuting following queries in access:
create table testbug (Field1 varchar (255) NULL)
insert into testbug (Field1) values ('a')
insert into testbug (Field1) values ('')
insert into testbug (Field1) values ('c')
select min(field1) from testbug
To my opinion this is a bug in ms-access. When the MIN function in ms-access comes across an empty string ('') it forgets all the values he has come across and returns the minimum value from all the values below the empty string. (in my simple example only value 'c')