Merge, delete when not on Source - sql

Question

I have an issue with a merge statement into a FACT TABLE
It was pretty simple until the users started to delete records from the source.
Current SQL:
Set Count = 1
WHEN NOT MATCHED
INSERT
WHEN MATCHED
UPDATED
New SQL:
So in this example, a record has been deleted from the source, it no longer matches but there is nothing to insert. I would like it the count to be set to 0.
WHEN DELETED FROM SOURCE
Set Count = 0
.
Source
Bob Jones | 1111
Mary Jones | 1112
James Jones | 1113
Helen Jones | 1114
TARGET
Bob Jones | 1111 | Count 1
Mary Jones | 1112| Count 1
James Jones | 1113| Count 1
Helen Jones | | 1114| Count 1
Peter Market | 1115| Count 0
I’m loading to a fact table using a merge and now they are just blanket deleting records, my facts are off. This must be accounted for somehow?
Thank you for any help at all.

Answer 1

You could do a merge using the target full outer joined with source, but id has to a unique key across both for this to work.
MERGE INTO target tgt
USING (SELECT CASE
WHEN s.id IS NULL THEN t.name --write this logic for all other non id columns.
ELSE s.name
END AS name,
coalesce(s.id, t.id) AS id, --use target's id when no source id is available
CASE
WHEN s.id IS NULL THEN 0 --zero for non matching ids
ELSE 1
END AS source_count
FROM target t
full outer join source s
ON s.id = t.id) src
ON ( src.id=tgt.id)
WHEN matched THEN
UPDATE SET tgt.name = src.name,
tgt.source_count = src.source_count
WHEN NOT matched THEN
INSERT (id,
name,
source_count)
VALUES(src.id,
src.name,
1) ; --insert 1 by default for new rows
Demo