T-SQL Insert into in the BEGIN CATCH - sql-server

Question

Thanks in advance for the help.
What I'm trying to achieve is handling the constraint violation of the FK (Municipality code) and when that's the case I want to insert the record in a fallout table.
In this block of code there result of the select can be null and therefore will throw an exception as I have a not null clause on the target.
TARGET.MUNICIPALITYCODE = (SELECT m.MUNICIPALITYCODE FROM MUNICIPALITY m WHERE SOURCE.MUNICIPALITYCODE = m.MUNICIPALITYCODE)
I wanted to be able to treat the exception on the BEGIN CATCH block and INSERT into a table of my choice the values that I was using on the SOURCE.
Does anyone knows if it is possible?
CREATE PROCEDURE upsertStagingToStreet
AS
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRY
BEGIN TRAN
MERGE STREET AS TARGET
USING STREET_STAGING AS SOURCE
ON (TARGET.correlationkey = SOURCE.correlationkey)
WHEN MATCHED
THEN UPDATE SET TARGET.Qty = SOURCE.Qty,
TARGET.MUNICIPALITYCODE = (SELECT m.MUNICIPALITYCODE FROM MUNICIPALITY m WHERE SOURCE.MUNICIPALITYCODE = m.MUNICIPALITYCODE)
WHEN NOT MATCHED BY TARGET
THEN INSERT (MUNICIPALITYCODE, STREENAME,STREECODE) VALUE ((SELECT m.MUNICIPALITYCODE FROM MUNICIPALITY m WHERE SOURCE.MUNICIPALITYCODE = m.MUNICIPALITYCODE),SOURCE.STREETCODE,SOURCE.STREETCODE)
COMMIT
END TRY
begin catch
# I'm not able to figure this part out.
INSERT INTO STREET_FALLOUT (MUNICIPALITYCODE, STREENAME,STREECODE,ERRORREASON) VALUES (SOURCE.MUNICIPALITYCODE,STREETNAME,STREETCODE,ERROR_MESSAGE())
end catch

Answer 1

You are approaching this wrong. It is not possible to insert some of the rows and catch errors on others.
Instead, just query the non-matching rows, and merge only the matching ones.
Note the lack of error-handling, and the inclusion of XACT_ABORT ON. This is the correct way, as all errors will cause the transaction to rollback anyway.
Note the SOURCE table in the merge is pre-joined with MUNICIPALITY, so only matching rows can appear.
A MERGE statement must have a semi-colon terminator, which is good practice anyway.
CREATE PROCEDURE upsertStagingToStreet
AS
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET XACT_ABORT, NOCOUNT ON;
BEGIN TRAN
INSERT INTO STREET_FALLOUT
(MUNICIPALITYCODE, STREENAME, STREECODE, ERRORREASON)
SELECT
SOURCE.MUNICIPALITYCODE,
SOURCE.STREETCODE,
SOURCE.STREETCODE,
'Missing MUNICIPALITYCODE'
FROM STREET_STAGING AS SOURCE
WHERE NOT EXISTS (SELECT 1
FROM MUNICIPALITY m
WHERE SOURCE.MUNICIPALITYCODE = m.MUNICIPALITYCODE
);
WITH SOURCE AS (
SELECT SOURCE.*
FROM STREET_STAGING AS SOURCE
JOIN MUNICIPALITY m ON SOURCE.MUNICIPALITYCODE = m.MUNICIPALITYCODE
)
MERGE STREET AS TARGET
USING SOURCE
ON (TARGET.correlationkey = SOURCE.correlationkey)
WHEN MATCHED THEN
UPDATE SET
Qty = SOURCE.Qty,
MUNICIPALITYCODE = SOURCE.MUNICIPALITYCODE
WHEN NOT MATCHED BY TARGET THEN
INSERT (MUNICIPALITYCODE, STREENAME, STREECODE)
VALUES (SOURCE.MUNICIPALITYCODE, SOURCE.STREETCODE, SOURCE.STREETCODE)
;
COMMIT;
GO