Empty dropdown list when “Null” value appears in the table

[MarcinM]

Hello,

There are the following tables (one-many relationship):

tblMeasuredSignals
ID_MeasSignal (PK)
(…other fields…)
intSignalID

tblSignalIDs
intSignalID (PK)

User assigns a number to a measured signal from a dropdown list (i.e. a cable connection must be made to measure a signal therefore each wire must have a unique ID). The idea is to hide these values from the dropdown list, which have already been assigned. This works fine using the following SQL statement:

SELECT tblSignalIDs.intSignalID
FROM tblSignalIDs
WHERE (((tblSignalIDs.intSignalID) Not In (SELECT tblMeasuredSignals.intSignalID FROM tblMeasuredSignals)))
ORDER BY tblSignalIDs.intSignalID;

PROBLEM:
Some signals can be measured using software (no cable needed => no ID for a cable needed => only signal description appears on the list and ID=null).
In case at least one “Null” value appears in [tblMeasurement].[intSignalID] then combobox using the above SQL statement is blank.

QUSTION:
Why does this happen and how can it be avoided?

[jforbes]

I'm not sure what's going on here, but if you are sure that it's a Null value that is messing things up, you can exclude them from the Select:

WHERE (((tblSignalIDs.intSignalID) Not In (SELECT tblMeasuredSignals.intSignalID FROM tblMeasuredSignals WHERE NOT tblMeasuredSignals.intSignalID IS NULL)))