trigger to only pass on real changes to the audit table - sql

Question

I wish to only pass on changes made to ProjectID to the audit table . If for instance changes are made , but the valve stays the same , then a record is not added to the audit table .
CREATE TRIGGER trgAfterUpdate ON [dbo].[Assets]
FOR UPDATE
AS
declare #assetid int;
declare #assetname nvarchar(max);
declare #projectid int;
declare #audit_action varchar(100);
select #assetid=i.AssetID from inserted i;
select #assetname=i.AssetName from inserted i;
select #projectid=i.ProjectID from inserted i;
if update(ProjectID)
set #audit_action='Updated Record -- After Update Trigger.';
insert into Asset_Test_Audit(AssetID,AssetName,Projectid,Audit_Action,Audit_Timestamp)
values(#assetid,#assetname,#projectid,#audit_action,getdate());
PRINT 'AFTER UPDATE Trigger fired.'
GO

Answer 1

you have to compare the results from inserted table with the deleted table like below:
CREATE TRIGGER trgAfterUpdate ON [dbo].[Assets]
FOR UPDATE
AS
declare #assetid int;
declare #assetname nvarchar(max);
declare #projectid int;
declare #audit_action varchar(100);
select #assetid=i.AssetID from inserted i;
select #assetname=i.AssetName from inserted i;
select #projectid=i.ProjectID from inserted i;
if update(ProjectID) and exists (select * from deleted
where AssetID<>#assetid or AssetName<>#assetname or ProjectID<>#projectid)
begin
set #audit_action='Updated Record -- After Update Trigger.';
insert into Asset_Test_Audit(AssetID,AssetName,Projectid,Audit_Action,Audit_Timestamp)
values(#assetid,#assetname,#projectid,#audit_action,getdate());
PRINT 'AFTER UPDATE Trigger fired.'
end
GO

Answer 2

You might want to do it a bit differently. Inserted and deleted tables hold previous and current states of ALL records updated, and you need to audit them all:
CREATE TRIGGER trgAfterUpdate ON [dbo].[Assets]
AFTER UPDATE
AS
set NoCount ON -- So that your insert does not mess with update rowcount
if update(ProjectID)
begin
insert into Asset_Test_Audit(AssetID, AssetName, Projectid, Audit_Action, Audit_Timestamp)
select AssetID, AssetName, ProjectID, 'Updated Record -- After Update Trigger.', getdate()
from Inserted
where not exists (select null from Deleted where Deleted.AssetID = Inserted.AssetID and Deleted.ProjectID = Inserted.ProjectID)
end
PRINT 'AFTER UPDATE Trigger fired.'
GO
A few pointers: do data retrieval in one select; instead of three selects that retrieve columns from Inserted, use one:
select #assetid=i.AssetID, #assetname=i.AssetName, #projectid=i.ProjectID from inserted i;
update(ColumnName) does not imply that value of that column has changed, but merely states that it participated in update:
update assets set ProjectID = ProjectID
still returns true in if update(ProjectID).
Use begin..end even when not necessary as it will save you a headache down the road. Sadly you will not know about the save, but you will certainly know (and feel the pain) if you don't.