T-SQL Merge from multiple source records - sql-server

Question

I've seen several similar questions but haven't found one that answers my question.
I have a source table with many individual notes for a company
CompanyName, Notes3
Company1, "spoke with someone"
Company2, "Email no longer works"
Company1, "Moved address"
I have a destination table (vchCompanyName is unique here)
vchCompanyName, vchNotes
Company1, "started business in 2005"
Company2, null
I want to end up with
vchCompanyName, vchNotes
Company1, "started business in 2005
spoke with someone
Moved address"
Company2, "Email no longer works"
I have tried this code
WITH CTE
AS (SELECT ROW_NUMBER() OVER (PARTITION BY CompanyName, Notes3 ORDER BY CompanyName) RowNum, *
FROM CompanyContact
)
merge dCompany as target
using CTE as source
on target.vchcompanyname = source.companyname
when matched and len(source.notes3)>0 and source.RowNum = 1
then
update set target.vchnote = vchnote + CHAR(13) + source.Notes3
But get the 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.
Which is accurate.
I have also tried STRING_AGG but get an undefined UDF error.
How do I change my code to run iteratively?
--EDIT--
I had tried the following update code
WITH CTE
AS (SELECT ROW_NUMBER() OVER (PARTITION BY CompanyName, Notes3 ORDER BY CompanyName) RowNum, *
FROM CompanyContact
)
UPDATE dCompany SET vchNote = vchNote +
(select CHAR(13) + cc.Notes3 from CompanyContact cc
inner JOIN dCompany dc ON dc.vchCompanyName COLLATE database_default = LEFT(cc.CompanyName,50) COLLATE database_default
inner join CTE on dc.vchCompanyName COLLATE database_default = LEFT(CTE.CompanyName,50) COLLATE database_default
WHERE LEN(cc.Notes3)>0
and RowNum = 1
);
But get the error
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Answer 1

#Chris Crawshaw, I will approach this by doing a 'union all' on the source and destination table to pick up all the notes for each company. Then using the STUFF function, it is easy to concatenate all the notes into one cell, while grouping by the induvidual company names. See the mockup below:
DECLARE #Source TABLE (CompanyName VARCHAR(20), Notes3 VARCHAR(50))
INSERT INTO #Source
SELECT 'Company1', 'spoke with someone' UNION ALL
SELECT 'Company2', 'Email no longer works' UNION ALL
SELECT 'Company1', 'Moved address'
DECLARE #Destination TABLE (vchCompanyName VARCHAR(20), vchNotes VARCHAR(500))
INSERT INTO #Destination
SELECT 'Company1', 'started business in 2005' UNION ALL
SELECT 'Company2', NULL
;WITH Temp AS (
SELECT *
FROM
(
SELECT *
FROM
#Destination D
WHERE D.vchNotes is not null
UNION ALL
SELECT *
FROM
#Source S
)h
)
update D
SET D.vchNotes=U.vchNotes
FROM #Destination D
LEFT JOIN(
SELECT t2.vchCompanyName, vchNotes=STUFF((
SELECT ',' + vchNotes
FROM Temp t1 where t1.vchCompanyName=t2.vchCompanyName
FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 1, 1, '')
FROM
#Destination t2
GROUP BY
t2.vchCompanyName
)U ON
U.vchCompanyName=D.vchCompanyName
--TEST--
SELECT *
FROM
#Destination