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:
Expand|Select|Wrap|Line Numbers
- SELECT tblSignalIDs.intSignalID
- FROM tblSignalIDs
- WHERE (((tblSignalIDs.intSignalID) Not In (SELECT tblMeasuredSignals.intSignalID FROM tblMeasuredSignals)))
- ORDER BY tblSignalIDs.intSignalID;
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?