Query where NOT NULL but only if NOT used as a FK - postgresql

Question

area
-----
id BIGSERIAL PRIMARY KEY
deleted_at TIMESTAMP WITH TIME ZONE DEFAULT NULL
and
registration
-----
area_id BIGINT REFERENCES area(id) NOT NULL
I want to get all records from area which have deleted_at IS NULL and the ones that can have deleted_at is NOT NULL but are present as a FK in the registration.
SELECT * FROM area
JOIN registration AS reg
ON reg.area_id=area.id
WHERE area.deleted_at IS NULL;
will omit the area records which are FKs in registration but have been marked as "deleted".
Adding an AND clause regarding the deleted_at column in the JOIN ON clause doesn't make sense, since it will only strip out valid records.
I can't quite wrap it around my head, since the two where conditions kind of contradict each other.

Answer 1

Try something like this:
SELECT *
FROM area
LEFT JOIN registration AS reg ON reg.area_id = area.id
WHERE (area.deleted_at IS NULL) <> (reg.area_id IS NOT NULL)
The LEFT JOIN would list all area rows, even without a matching row from registration. (Resulting NULL values for those rows.)
The WHERE clause makes sure that both of the fields are not NULLs at the same time.

Answer 2

I think this is what your asking for. When you use left join it data fields for registration will show up as null where they are not present in registration table.
select * from area
left join registration as reg
on reg.area_id= area.id
where area.deleted_at is null or reg.area_id is not null;

Answer 3

-- I need (0) all area records EXCEPT the ones where (1) deleted_at IS NOT NULL
-- AND (2) are NOT present as FKs in registration.
SELECT * FROM area a
WHERE NOT(
a.deleted_at IS NOT NULL -- (1)
AND NOT EXISTS (
SELECT * -- (2)
FROM registration r
WHERE r.area_id=a.id
)
);
Note: your textual phrasing is confusing: EXCEPT a AND b could mean two things
And, after the rephrasing of the question:
-- I want to get (0) all records from area (1) which have deleted_at IS NULL (1a)
-- and (2) the ones that can have deleted_at is NOT NULL but are present as a FK in the registration.
SELECT * FROM area a
WHERE a.deleted_at IS NULL -- (1)
OR a.deleted_at IS NOT NULL AND EXISTS ( (1a)
SELECT * -- (2)
FROM registration r
WHERE r.area_id=a.id
);
If I understand correctly, you mean plus the ones at (1a) : if so, the and in (1a) is translated into an or

Answer 4

Are you simply searching for the following query?
SELECT * FROM Area
LEFT OUTER JOIN registration on id = area_id
WHERE deleted_at IS NULL OR area_id IS NOT NULL
This will return the same area.id multiple times if registration.area_id is not unique though (since you have no UNIQUE constraints).
If that is a problem, you may want the following query instead.
SELECT * FROM Area
WHERE deleted_at IS NULL OR id IN (SELECT area_id FROM registration)
Or this, built with a COUNT:
SELECT id, deleted_at, COUNT(*) FROM Area
LEFT OUTER JOIN registration on id = area_id
WHERE (deleted_at IS NULL or area_id IS NOT NULL)
GROUP BY id, deleted_at