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Using MERGE for Audting


Using MERGE for Audting

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terry999
terry999
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Hi
I'm looking at using Merge which I've never used. I've achieved Auditing before with an After trigger that writes the old row to the audit table. I'm only interested in writting to the audit table if the values change so use checksum.


create table tblCustomer
(CID int not null primary key,
name varchar(10) not null,
country varchar(15) not null)

insert into tblCustomer values (1, 'Bob', 'UK'), (2, 'Claire', 'USA'), (3,'Terry','France')
create table tblUpdates
(
CID int not null,
name varchar(10) not null,
country varchar(15) not null
)

insert into tblUpdates values (1, 'Bob', 'UK'), (2, 'Claire', 'Mexico'), (3,'Terrance','Spain'),(4,'Abdul','Egypt')

select 'UPDATE' as actiontype, getdate() as AuditDate, name, country
into #tAudit from tblCustomer where 1=2

insert into #tAudit
SELECT Actiontype, AuditDate, name, country FROM
(
MERGE tblCustomer AS target
USING (SELECT * FROM tblUpdates ) AS source
ON (target.cid = source.cid)

WHEN MATCHED
THEN UPDATE SET name = source.name, country = source.country
WHEN NOT matched
THEN INSERT
VALUES (CID,Name, Country)

OUTPUT $action as Actiontype, getdate() as AuditDate,
CASE WHEN $action = 'INSERT' THEN inserted.name ELSE deleted.name END as name,
CASE WHEN $action = 'INSERT' THEN inserted.country ELSE deleted.country END as country,
binary_checksum(inserted.name, inserted.country) as chkNew,
binary_checksum(deleted.name, deleted.country) as ChkOld) as tChanges
WHERE chkNew <> ChkOld

select * from #tAudit

drop table #tAudit
drop table tblCustomer
drop table tblUpdates



Is there a better way to achive this.
Jason-299789
Jason-299789
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You can use the OUTPUT to insert rows into a table rather than the Outer Select, which is rather cumbersome.

this might be better, and you'd need to double check the code works but theres no reason it shouldnt



MERGE tblCustomer AS target
USING (SELECT * FROM tblUpdates ) AS source
ON (target.cid = source.cid)

WHEN MATCHED
AND (target.name!=source.name
or target.country!=source.country)
THEN UPDATE SET name = source.name, country = source.country
WHEN NOT matched
THEN INSERT
VALUES (CID,Name, Country)

OUTPUT $action
, getdate()
, CASE WHEN $action = 'INSERT' THEN inserted.name ELSE deleted.name END as name
, CASE WHEN $action = 'INSERT' THEN inserted.country ELSE deleted.country END as country
, binary_checksum(inserted.name, inserted.country) as chkNew
,binary_checksum(deleted.name, deleted.country) as ChkOld) as tChanges
INTO #tAudit
WHERE chkNew <> ChkOld



Edit I've just seen that you dont have a clause to update only the rows that have changed, so I've added it to the code this will then only update changed rows and insert new rows.

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ChrisM@Work
ChrisM@Work
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If you are using the Enterprise edition of SS2K8, then change data capture is well worth a look.

“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden
Exploring Recursive CTEs by Example Dwain Camps
terry999
terry999
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Thank you. Your example is easier to read.

BTW I was restricting to only chnage rows by comparing the checksum values.

I didn't know you could add additional filters after is matched i.e. target.name != source.name
Jason-299789
Jason-299789
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terry999 (10/2/2012)
Thank you. Your example is easier to read.

BTW I was restricting to only chnage rows by comparing the checksum values.

I didn't know you could add additional filters after is matched i.e. target.name != source.name



Sorry theres a coding error I believe it should read


MERGE tblCustomer AS target
USING (SELECT * FROM tblUpdates ) AS source
ON (target.cid = source.cid)

WHEN MATCHED
AND (target.name!=source.name
or target.country!=source.country)
THEN UPDATE SET name = source.name, country = source.country
WHEN NOT matched
THEN INSERT
VALUES (CID,Name, Country)

OUTPUT $action
, getdate()
, CASE WHEN $action = 'INSERT' THEN inserted.name ELSE deleted.name END
, CASE WHEN $action = 'INSERT' THEN inserted.country ELSE deleted.country END
, binary_checksum(inserted.name, inserted.country)
,binary_checksum(deleted.name, deleted.country)
INTO #tAudit



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terry999
terry999
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Thanks Jason

I had made that change.

From now on I will use MERGE to replace Triggers for audit - all in one place.
terry999
terry999
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illegal?
TheSQLGuru
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terry999 (10/2/2012)
Thanks Jason

I had made that change.

From now on I will use MERGE to replace Triggers for audit - all in one place.



Note that this places the onus on the developer to code the audit stuff for EVERY SINGLE DML OPERATION ON EACH TABLE. A trigger can be written once and will capture ALL DML operations, regardless of provenance. Even if your devs are rigorous, what about that quick "oopsie" data cleanup operation you need to do because someone screwed something up. Will you ALWAYS remember to do the MERGE, even for ad hoc DML?

Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service
terry999
terry999
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Good point, I'd forgotten that
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