SQL left join issue with null values - sql

Question

I inherited a query that uses left joins. One of the things the query is doing is removing any archived records, that is where archived = 'Y'. This is how the query looks:
select P.firstname, E.entityid, C.committeeid, L.locationname
from Pract P
left join Committee C
on P.commiteeid = C.committeeid
and c.archived = 'N'
left join Entity E
on P.entityid = E.EntityID
and e.archived = 'N'
left join Location L
on E.location = L.location
and l.archived = 'N'
The result should only return records where archived <> 'Y'. I think a problem with putting the filter with the "on" is that it will return a record where c.archived = 'N' and just put a null in the archived field, which is not correct:
FirstName EntityID CommitteeId
John 55 null
If c.archived = 'Y' then the record should not show up.
I believe the archived filter should be in the where clause, like this:
select firstname, entityid, committeeid
from Pract P
left join Committee C
on P.commiteeid = C.committeeid
left join Entity E
on P.entityid = E.entityid
left join Location L
on E.Locationid = L.locationid
where c.archived = 'N'
and e.archived = 'N'
and l.archived = 'N'
The problem I'm finding is that there are instances where the archived field from Committee is null(it's not a 'Y' or an 'N'). Using my solution incorrectly eliminates the records since null <> 'N.'
If I try this:
where c.archived <> 'Y'
it does not work, I'm guessing because NULL does not evaluate to anything.
If I try this:
where (c.archived = 'N' or c.archived is null)
it doesn't work as it now brings back those null records caused by the left join. I can't replace the left join with an inner join because that will exclude records where c.committeeid is null.
I just want to bring back records where archived <> 'Y', which includes those where the field is null.
To be clear, this is what the records in the table can look like:
FirstName EntityID Archived
John 55 Y
Tom 56 NULL
Rob 57 N
In this instance I want the returned records to look like:
Tom 56 NULL
Rob 57 N
John would be eliminated because Archived = 'Y.'
Is there another way to do this?

Answer 1

If I understand correctly, you want to use another field in the where clause for the filtering:
select firstname, entityid, committeeid -- this will return an error on committeeid
from Pract P left join
Committee C
on P.commiteeid = C.committeeid and
(c.archived <> 'Y' or c.archived is null)
where c.committeeid is not null;
You may also be able to do this with exists more easily:
select p.*
from Pract P
where not exists (select 1
from Committee C
where P.commiteeid = C.committeeid and
c.archived = 'Y'
);
This handles the NULL values automagically.

Answer 2

I believe that you are looking for this version. You just start with what you had and modify joining condition to cover values that are not equal to Y and that are NULL.
select firstname, entityid, committeeid
from Pract P
left join Committee C
on P.commiteeid = C.committeeid
and (c.archived <> 'Y' OR c.archived IS NULL)
Then if you need only items that has record in commitee table. you put it in WHERE clause.
WHERE P.commiteeeid = C.commiteeid
But it will just emulate INNER JOIN.
Your problem with comparing NULL is that you can't use <> to test for NULL values. If you want records with NULL values you have to test for IS NULL explicitly.
So your updated query would look like:
select P.firstname, E.entityid, C.committeeid, L.locationname
from Pract P
left join Committee C
on P.commiteeid = C.committeeid
and (c.archived <> 'Y' OR c.archived IS NULL)
left join Entity E
on P.entityid = E.EntityID
and (e.archived <> 'Y' OR e.archived IS NULL)
left join Location L
on E.location = L.location
and (l.archived <> 'Y' OR l.archived IS NULL)
And if you have only values 'Y' 'N' and NULL, then you can also use
and (c.archived = 'N' OR c.archived IS NULL)
Just to be sure we understand each other here is minimal example for your query
create table Pract (firstname varchar(20), cid int)
create table comt ( cid int, archived varchar(1) )
insert into Pract values ('Tom',1),('Adam',2),('Mark',3),('Bob',4)
insert into comt values (1,'Y'), (2,'N'), (3,NULL)
--
select firstname, P.cid, C.cid, C.archived
from Pract P
LEFT join comt C
on P.cid = C.cid
and (c.archived <> 'Y' OR c.archived IS NULL)
With result
firstname cid cid archived
1 Tom 1 NULL NULL
2 Adam 2 2 N
3 Mark 3 3 NULL
4 Bob 4 NULL NULL
Or with INNER JOIN
select firstname, P.cid, C.cid, C.archived
from Pract P
INNER join comt C
on P.cid = C.cid
and (c.archived <> 'Y' OR c.archived IS NULL)
With result
firstname cid cid archived
1 Adam 2 2 N
2 Mark 3 3 NULL

Answer 3

The solution depends on how you want to handle the NULL values from the left join. If NULL means not archived then
select firstname, entityid, committeeid
from Pract P
left join Committee C
on P.commiteeid = C.committeeid
left join Entity E
on P.entityid = E.entityid
left join Location L
on E.Locationid = L.locationid
where (c.archived = 'N' OR c.archived IS null)
and (e.archived = 'N' OR e.archived IS null)
and (l.archived = 'N' OR l.archived IS NULL)