Union Subquery produces Results Non-Unioned Subquery doesn't - sql

Question

I'm using Access 2010 and i'm having a very strange query result.
I've created a query that includes a union subquery that produces results. I've also created a query that has a non-unioned subquery that doesn't produce any records. It's very strange because the union subquery is the non-unioned subquery twice.
Non-Unioned Subquery:
SELECT NVP, [NVP NAME], MIN([CURRENT PERIOD]) AS [MIN PERIOD], [BASE PERIOD]
FROM AGGDATA
GROUP BY [NVP NAME],NVP,[BASE PERIOD]
Unioned Subquery:
SELECT NVP, [NVP NAME], MIN([CURRENT PERIOD]) AS [MIN PERIOD], [BASE PERIOD]
FROM AGGDATA
GROUP BY [NVP NAME],NVP,[BASE PERIOD]
UNION
SELECT NVP, [NVP NAME], MIN([CURRENT PERIOD]) AS [MIN PERIOD], [BASE PERIOD]
FROM AGGDATA
GROUP BY [NVP NAME],NVP,[BASE PERIOD]
Here is the whole Query:
SELECT sub.NVP,sub.[NVP NAME], SUB.[MIN PERIOD], SUB.[BASE PERIOD], TT.[ACTIVE PERIODS]
FROM
(SELECT AGGD.NVP, AGGD.[NVP NAME], MIN(AGGD.[CURRENT PERIOD]) AS [MIN PERIOD], [BASE PERIOD]
FROM AGGDATA AGGD
GROUP BY AGGD.[NVP NAME],AGGD.NVP,[BASE PERIOD]
UNION
SELECT AGGD.NVP, AGGD.[NVP NAME], MIN(AGGD.[CURRENT PERIOD]) AS [MIN PERIOD], [BASE PERIOD]
FROM AGGDATA AGGD
GROUP BY AGGD.[NVP NAME],AGGD.NVP,[BASE PERIOD])
AS SUB
INNER JOIN
(SELECT SUB.NVP,
SUB.[BASE PERIOD],
COUNT([CURRENT PERIOD]) AS [ACTIVE PERIODS]
FROM
(SELECT DISTINCT NVP, [BASE PERIOD], [CURRENT PERIOD]
FROM AGGDATA)
AS SUB
GROUP BY SUB.NVP, SUB.[BASE PERIOD]
having COUNT([CURRENT PERIOD]) < 10)
AS TT
ON SUB.[BASE PERIOD] = TT.[BASE PERIOD] AND SUB.NVP = TT.NVP
WHERE SUB.[MIN PERIOD] > TT.[BASE PERIOD]
Does anyone know why the Union subquery works but the non-unioned subquery doesn't?
EDIT: The two subqueries do produce the same results, the issue is with the whole query.
Thank you!

Answer 1

I think you have some issues with your parentheses and aliases. Try this code instead:
SELECT
mn.NVP,[NVP NAME], [MIN PERIOD], mn.[BASE PERIOD], TT.[ACTIVE PERIODS]
FROM
(SELECT
[NVP NAME],NVP,[BASE PERIOD], MIN([CURRENT PERIOD]) AS [MIN PERIOD]
FROM
AGGDATA
GROUP BY
[NVP NAME],NVP,[BASE PERIOD]) AS mn INNER JOIN
(SELECT
NVP,
[BASE PERIOD],
COUNT([CURRENT PERIOD]) AS [ACTIVE PERIODS]
FROM
(SELECT DISTINCT
NVP, [BASE PERIOD], [CURRENT PERIOD]
FROM
AGGDATA) AS a
GROUP BY
NVP, [BASE PERIOD]
HAVING
COUNT([CURRENT PERIOD]) < 10) AS TT ON
mn.[BASE PERIOD] = TT.[BASE PERIOD] AND
mn.NVP = TT.NVP
WHERE
mn.[MIN PERIOD] > TT.[BASE PERIOD]