Distinct Aggregate Query in Microsoft Access with Group By on a Different Field - sql

Question

Among the fields in my table are 2 fields, CTN_NUM and PO_NUM. Each PO_NUM has at least one CTN_NUM, possibly more—distinct or repeated. Any given CTN_NUM cannot have more than 1 PO_NUM. In other words, it's a one to many relationship. I want to create a query which shows the number of unique CTN_NUM's per PO_NUM. I've seen other threads on this forum as well as http://blogs.office.com/b/microsoft-access/archive/2007/09/19/writing-a-count-distinct-query-in-access.aspx and none seem to address this exact issue.
Here's what I tried:
A)
SELECT PO_NUM, Count(CTN_NUM) AS CountOfCTN_NUM
FROM tempSpring_ASN
GROUP BY PO_NUM;
This returns the count of ALL CTN_NUMs per PO_NUM, even if they are not unique.
B)
SELECT PO_NUM, Count(DISTINCT CTN_NUM) AS CountOfCTN_NUM
FROM tempSpring_ASN
GROUP BY PO_NUM;
While this may work in other RDBMS’s, in Access I get a syntax error.
C)
SELECT COUNT(*)
FROM
(SELECT DISTINCT CTN_NUM AS cn
FROM tempSpring_ASN);
This returns the sum of CTN_NUM’s which are distinct to the table, not distinct to the PO_NUM.
D) Same as C, but with a GROUP BY:
SELECT COUNT(*)
FROM
(SELECT DISTINCT CTN_NUM AS cn
FROM tempSpring_ASN)
GROUP BY PO_NUM;
This prompts me for the PO_NUM.
Can you please advise? Thanks.

Answer 1

Try this one:
SELECT PO_NUM, COUNT(*) AS CountOfCTN_NUM
FROM (
SELECT PO_NUM, CTN_NUM
FROM tempSpring_ASN
GROUP BY PO_NUM, CTN_NUM
) a
GROUP BY PO_NUM;
This should also work:
SELECT PO_NUM, COUNT(*) AS CountOfCTN_NUM
FROM (
SELECT DISTINCT PO_NUM, CTN_NUM
FROM tempSpring_ASN
) a
GROUP BY PO_NUM;

Answer 2

Letter D has a syntax error on it. PO_NUM is not present on subquery so you can't use group by on it. It might be that the subquery misses PO_NUM.