using criteria in an update query involving a join - sql

Question

I'm using MS Access
The SQL below updates the CurrNumTees field in the Parent tblContact records with the number of tblTorTee records that have an end date (which is not the ultimate effect I am aiming for, but I provide it as a starting point.
UPDATE tblContact
INNER JOIN tblTorTee ON tblContact.ContactId = tblTorTee.TorId
SET tblContact!CurNumTees = DCount("[tblTorTee.EndDate]",
"tbltortee","Torid = " & [ContactId]);
I need to update the CurrNumTees field with the number of records in tblTorTee that do not have an EndDate, in other words, that field is blank. I’ve tried using WHERE and HAVING and IS NULL in various combinations and locations, but without success. Could you help point me in the right direction?

Answer 1

The MS Access COUNT function does not count nulls, so I think you have to do this in two stages.
Firstly create a query like this:
SELECT TorId, IIF(ISNULL(EndDate),1,0) AS isN
FROM tblTorTee
WHERE EndDate IS NULL;
And save it as QryEndDateNull
Now you can run an Update Query like this:
UPDATE tblContact
SET tblContact.CurNumTees = DSUM("IsN","QryEndDateNull","TorId = " & [ContactID]);

Answer 2

Saving calculated data (data dependent on other data) is usually a bad design, especially aggregate data. Should just calculate when needed.
Did you try the IS NULL criteria within the DCount()?
UPDATE tblContact Set CurNumTees = DCount("*", "tblTorTee", "EndDate Is Null AND TorId = " & [ContactId]);