SQL Merge Error : The MERGE statement attempted to UPDATE or DELETE [duplicate]

Question

I have a source table

select 54371 Id, 'foo' [CreateBy], '2016-10-24 09:29:18.548'[CreateDate], 'foo'[UpdateBy],  '2016-10-24 09:29:18.548'[UpdateDate], 'E'[MT], 185761[MID], 3[BGID]
union
select 54372, 'foo', '2016-10-24 09:30:18.548', 'foo',  '2016-10-24 09:30:18.548',  'E', 185761, 2

and a target table

select 54379 Id, 'foo' [CreateBy], '2016-10-24 09:29:18.548'[CreateDate], 'foo'[UpdateBy],  '2016-10-24 10:29:18.548'[UpdateDate], 'E'[MT], 185761[MID], 3[BGID]

What I want is to match based on MT, MID and

1. Insert if not exists
2. update if BGID matches
3. delete if BGID does not matches

When I use SQL Merge Statement I get error

The MERGE statement attempted to UPDATE or DELETE the same row more than once. This happens when a target row matches more than one source row. A MERGE statement cannot UPDATE/DELETE the same row of the target table multiple times. Refine the ON clause to ensure a target row matches at most one source row, or use the GROUP BY clause to group the source rows.`

My Merge is like this

MERGE   
  FooBar AS target
USING
(
SELECT
E.[Id],
E.[CreateBy],
E.[CreateDate],
E.[UpdateBy],
E.[UpdateDate],
E.[MT],
E.[MID],
E.[BGID]
FROM @FooBar E
) AS source
ON
source.MID = target.MID
AND source.MT = target.MT
WHEN MATCHED and target.[BGID] = source.[BGID] THEN
UPDATE SET
target.[UpdateBy] = Source.[UpdateBy]
,target.[UpdateDate] = Source.[UpdateDate]
When Matched and source.BGID <> target.BGID THEN
DELETE
WHEN NOT MATCHED THEN
INSERT([CreateBy]
,[CreateDate]
,[UpdateBy]
,[UpdateDate]
,[MT]
,[MID]
,[BGID])
VALUES
(
Source.[CreateBy]
,Source.[CreateDate]
,Source.[UpdateBy]
,Source.[UpdateDate]
,Source.[MT]
,Source.[MID]
,Source.[BGID]
);

What am I missing?

Answer 1

You are joining the tables on ON source.MappingId = target.MappingId.

In your data sample, there are more than 1 row with same MappingId = 185761. So here you got:

A MERGE statement cannot UPDATE/DELETE the same row of the target table multiple times.

You need to specify some unique column combination to join the source and the target tables.