Access 2010 Query Confusion - sql

Question

Just need some quick clarification
I have 2 Queries in my Access Database that should return Inverse results:
SELECT Equipment.title
FROM Equipment
WHERE (((Equipment.[EquipmentID]) Not In (
select EquipmentID
from DownPeriod
where UpDate is null
)));
The 2nd just excludes the Not before the In.
My Confusion comes from the fact that the query posted above does not return any results if an EquipmentID field has at least 1 null value in the DownPeriod table.
It works fine if the fields are filled, and the inverse query list always works. This makes me think there's an issue with the null value.
Now this field should never be null but I wanted to know if I could still get this to work in the unlikely event a null did occur.
Thank you in advanced!

Answer 1

Try joins:
SELECT Equipment.title FROM Equipment INNER JOIN DownPeriod
ON Equipment.EquipmentID = DownPeriod.EquipmentID
WHERE DownPeriod.UpDate is null
and
SELECT Equipment.title FROM Equipment INNER JOIN DownPeriod
ON Equipment.EquipmentID = DownPeriod.EquipmentID
WHERE DownPeriod.UpDate is not null
See if a change in syntax fixes your issue.
Not only should this work, but I believe it is a faster practise than using the IN() NOT IN() methods (might be wrong on that, but it looks nicer to read). It also adds the ability to quickly change the "is not null" criteria just the same as IN->NOT IN

Answer 2

I agree with StuckAtWork's approach. However, if you still want to understand why your original approach didn't produce the results you want, I think I can help you.
There may be an issue with empty strings which could complicate the situation. But regardless of whether or not empty strings are involved you have something more fundamental to consider.
Here is my version of the Equipment table.
EquipmentID title
1 one
2 two
3 three
And here is my version of the DownPeriod table.
ID EquipmentID text_field
1 1 one
2 2 two
3 Null
4 3 three
I didn't include your UpDate field in my DownPeriod table. It's irrelevant to your problem.
I pasted your SQL into a new Access query, discarded the WHERE clause from the subquery, and got exactly the same result as this query --- no rows returned:
SELECT e.title
FROM Equipment AS e
WHERE
e.EquipmentID Not In (
SELECT EquipmentID
FROM DownPeriod
);
So consider this situation from the db engine's perspective. Using my version of the Downloads table, it has a set of values (1, 2, Null, and 3) from the subquery. You're asking it to show you the rows from Equipment where EquipmentID is NOT IN that list of values. The db engine will only give you the rows for which that condition is True.
Null is the problem. For each EquipmentID, when it considers whether that value is not present in the subquery set, it doesn't know. That Null is an unknown value ... and the unknown value might be the same as the current EquipmentID it's considering ... or might be something else. But since the db engine doesn't know the real value, it can't evaluate the condition as True, so will not include that row in the result set. The same thing happens for every row in Equipment table ... therefore your query's result set is empty (no rows).
You could get your desired results by excluding Null values from the subquery result set with a WHERE clause like the one below. But I think StuckAtWork's suggestion is a better way to go.
SELECT e.title
FROM Equipment AS e
WHERE
e.EquipmentID Not In (
SELECT EquipmentID
FROM DownPeriod
WHERE EquipmentID Is Not Null
);