Values Disappear when Filtering Correlated Subquery - sql

Question

This question is related to the recent answer I provided here.
Setup
Using MS Access 2007.
Assume I have a table called mytable consisting of three fields:
id Long Integer AutoNumber (PK)
type Text
num Long Integer
With the following sample data:
+----+------+-----+
| id | type | num |
+----+------+-----+
| 1 | A | 10 |
| 2 | A | 20 |
| 3 | A | 30 |
| 4 | B | 40 |
| 5 | B | 50 |
| 6 | B | 60 |
| 7 | C | 70 |
| 8 | C | 80 |
| 9 | C | 90 |
| 10 | D | 100 |
+----+------+-----+
Similar to the linked answer, say I wish to output the three fields, with a running total for each type value, with the value of the running total limited to a maximum of 100, I might use a correlated subquery such as the following:
select q.* from
(
select t.id, t.type, t.num,
(
select sum(u.num)
from mytable u where u.type = t.type and u.id <= t.id
) as rt
from mytable t
) q
where q.rt < 100
This produces the expected result:
+----+------+-----+----+
| id | type | num | rt |
+----+------+-----+----+
| 1 | A | 10 | 10 |
| 2 | A | 20 | 30 |
| 3 | A | 30 | 60 |
| 4 | B | 40 | 40 |
| 5 | B | 50 | 90 |
| 7 | C | 70 | 70 |
+----+------+-----+----+
Observation
Now assume that I wish to filter the result to show only those values for type like "[AB]".
If I use either of the following queries:
select q.* from
(
select t.id, t.type, t.num,
(
select sum(u.num)
from mytable u where u.type = t.type and u.id <= t.id
) as rt
from mytable t
where t.type like "[AB]"
) q
where q.rt < 100
select q.* from
(
select t.id, t.type, t.num,
(
select sum(u.num)
from mytable u where u.type = t.type and u.id <= t.id
) as rt
from mytable t
) q
where q.rt < 100 and q.type like "[AB]"
The results are filtered as expected, but the values in the rt (running total) column disappear:
+----+------+-----+----+
| id | type | num | rt |
+----+------+-----+----+
| 1 | A | 10 | |
| 2 | A | 20 | |
| 3 | A | 30 | |
| 4 | B | 40 | |
| 5 | B | 50 | |
+----+------+-----+----+
Question
Why would the filter cause the values returned by the correlated subquery to disappear?
Thank you for your time reading my question and in advance for any advice you can offer.

Answer 1

Moving type criteria to the aggregate subquery works.
One less tier works but the aggregate subquery has to repeat in WHERE clause:
SELECT mytable.*, (select sum(u.num)
from mytable u where u.type = MyTable.type and u.id <= MyTable.id
) AS rt
FROM mytable
WHERE ((((select sum(u.num)
from mytable u where u.type = MyTable.type and u.id <= MyTable.id
))<100) AND ((mytable.[type]) Like "[AB]"));
An INNER JOIN version:
select MyTable.*, q.* from MyTable INNER JOIN
(
select t.id, t.type, t.num,
(
select sum(u.num)
from mytable u where u.type = t.type and u.id <= t.id
) as rt
from mytable t
) q
ON q.id=MyTable.ID
where q.rt < 100 AND MyTable.Type LIKE "[AB]";