Issue with delete statement in merge statement - sql-server

Question

I am working with merge into statement
I have a table that looks like this (first I insert like this) using below query:
5
2
5
3
5
5
5
6
table type :
CREATE TYPE [dbo].[userid] AS TABLE(
[userid] [bigint] NULL
)
GO
Now I want the below output :
5
2
5
3
5
6
I write the below query like this:
--use test
declare #sid varchar(100) = '5'
declare #uid as userid
insert into #uid(userid) values(2)
insert into #uid(userid) values(3)
--insert into #uid(userid) values(5) // I remove this line
insert into #uid(userid) values(6)
MERGE INTO dbo.test_master AS dest
USING #uid AS sou ON
dest.sid = #sid
AND
sou.userid = dest.testid
WHEN MATCHED THEN UPDATE SET
dest.testid = sou.userid
WHEN NOT MATCHED THEN
INSERT( sid, testid )
VALUES( #sid, sou.userid )
--WHEN NOT MATCHED BY SOURCE
-- THEN
-- DELETE
;
I am trying to achieve this output
5
2
5
3
5
6
I am using delete keyword, see my SQL query, but it is deleting the all records from the table. I try, but can't work it out.

Answer 1

You need to pre-filter the destination table, otherwise all rows, even ones that have a different sid will be deleted. You can pre-filter with a CTE or a view.
I note that the WHEN MATCHED clause makes no sense, as the only column being updated is the join column, which obviously matches anyway.
declare #sid varchar(100) = '5';
declare #uid as userid;
insert into #uid (userid) values
(2),
(3),
-- (5), -- I remove this line
(6);
WITH dest AS (
SELECT *
FROM dbo.test_master dest
WHERE dest.sid = #sid
)
MERGE INTO dest
USING #uid AS sou ON
sou.userid = dest.testid
-- WHEN MATCHED THEN UPDATE SET
-- dest.testid = sou.userid -- doesn't make sense
WHEN NOT MATCHED THEN
INSERT( sid, testid )
VALUES( #sid, sou.userid )
WHEN NOT MATCHED BY SOURCE THEN
DELETE
;
db<>fiddle