Merge - Only update if values have changed - sql

Question

I am running a merge in SQL Server. In my update, I want to only update the row if the values have changed. There is a version row that increments on each update. Below is an example:
MERGE Employee as tgt USING
(SELECT Employee_History.Emp_ID
, Employee_History.First_Name
, Employee_History.Last_Name
FROM Employee_History)
as src (Emp_ID,First_Name,Last_Name)
ON tgt.Emp_ID = src.Emp_ID
WHEN MATCHED THEN
UPDATE SET
Emp_ID = src.Emp_ID,
,[VERSION] = tgt.VERSION + 1
,First_Name = src.First_Name
,Last_Name = src.Last_Name
WHEN NOT MATCHED BY target THEN
INSERT (Emp_ID,0,First_Name,Last_Name)
VALUES
(src.Emp_ID,[VERSION],src.First_Name,src.Last_Name);
Now, if I only wanted to update the row, and thus increment version, ONLY if the name has changed.

Answer 1

WHEN MATCHED can have AND . Also, no need to update EMP_ID .
...
WHEN MATCHED AND (trg.First_Name <> src.First_Name
OR trg.Last_Name <> src.Last_Name) THEN UPDATE
SET
[VERSION] = tgt.VERSION + 1
,First_Name = src.First_Name
,Last_Name = src.Last_Name
...
If Last_Name or First_Name are nullable, you need to take care of NULL values while comparing trg.Last_Name <> src.Last_Name , for instance ISNULL(trg.Last_Name,'') <> ISNULL(src.Last_Name,'')

Answer 2

The answer provided by a1ex07 is the right answer, but i just wanted to expand on the difficulty in comparing a large number of columns, watching for nulls, etc.
I found that I could generate a checksum in some CTE's with hashbytes, target those CTEs in the merge, and then use the "update and...." condition specified above to compare the hashes:
with SourcePermissions as (
SELECT 1 as Code, 1013 as ObjectTypeCode, 'Create Market' as ActionName, null as ModuleCode, 1 as AssignableTargetFlags
union all SELECT 2, 1013, 'View Market', null, 1
union all SELECT 3, 1013, 'Edit Market', null, 1
--...shortened....
)
,SourcePermissions2 as (
select sp.*, HASHBYTES('sha2_256', xmlcol) as [Checksum]
from SourcePermissions sp
cross apply (select sp.* for xml raw) x(xmlcol)
)
,TargetPermissions as (
select p.*, HASHBYTES('sha2_256', xmlcol) as [Checksum]
from Permission p
cross apply (select p.* for xml raw) x(xmlcol)
) --select * from SourcePermissions2 sp join TargetPermissions tp on sp.code=tp.code where sp.Checksum = tp.Checksum
MERGE TargetPermissions AS target
USING (select * from SourcePermissions2) AS source ([Code] , [ObjectTypeCode] , [ActionName] , [ModuleCode] , [AssignableTargetFlags], [Checksum])
ON (target.Code = source.Code)
WHEN MATCHED and source.[Checksum] != target.[Checksum] then
UPDATE SET [ObjectTypeCode] = source.[ObjectTypeCode], [ActionName]=source.[ActionName], [ModuleCode]=source.[ModuleCode], [AssignableTargetFlags] = source.[AssignableTargetFlags]
WHEN NOT MATCHED THEN
INSERT ([Code] , [ObjectTypeCode] , [ActionName] , [ModuleCode] , [AssignableTargetFlags])
VALUES (source.[Code] , source.[ObjectTypeCode] , source.[ActionName] , source.[ModuleCode] , source.[AssignableTargetFlags])
OUTPUT deleted.*, $action, inserted.[Code]
--only minor issue is that you can no longer do a inserted.* here since it gives error 404 (sql, not web), complaining about returning checksum which is included in the target cte but not the underlying table
,inserted.[ObjectTypeCode] , inserted.[ActionName] , inserted.[ModuleCode] , inserted.[AssignableTargetFlags]
;
Couple of notes: I could have simplified greatly with checksum or binary_checksum, but I always get collisions with those.
As to the 'why', this is part of an automated deployment to keep a lookup table up to date. The problem with the merge though is there is an indexed view that is complex and heavily used, so updates to the related tables are quite expensive.

Answer 3

Rather than avoiding an update altogether, you could change your [VERSION] + 1 code to add zero when names match:
[VERSION] = tgt.VERSION + (CASE
WHEN tgt.First_Name <> src.First_Name OR tgt.Last_Name <> src.Last_Name
THEN 1
ELSE 0 END)

Answer 4

#a1ex07 thanks for the answer.. a slight correction.. I am not following SQL version so this could be a change in SQL specification
WHEN MATCHED AND CONDITION THEN UPDATE
The above is not a valid syntax
Following is valid
WHEN MATCHED THEN UPDATE SET ... WHERE CONDITION WHEN NOT MATCHED THEN INSERT...
so would change it to
WHEN MATCHED THEN UPDATE
SET
[VERSION] = tgt.VERSION + 1
,First_Name = src.First_Name
,Last_Name = src.Last_Name
WHERE
trg.First_Name <> src.First_Name
OR trg.Last_Name <> src.Last_Name
https://docs.oracle.com/cd/B28359_01/server.111/b28286/statements_9016.htm#SQLRF01606