Creating an Audit trail on a table using a trigger

Hi,
This is kind of following on from my last couple of posts regarding Identity
columns and so on.
Basically, I want to ensure that for a particular table, every row has a
numeric reference. This reference must be unique and gapless. Ideally it
should order in the sequence of the records being inserted however this
isn't an absolute requirement.
From my (limited) understanding of SQLS, I think I can achieve this with a
FOR INSERT Trigger - in that the trigger is fired every time a row is
inserted and the trigger is the same transaction as the initial insert hence
I avoid any concurrency issues.
However I'm not completely sure how to achieve this. I think that my trigger
should be along the lines of this...
CREATE TRIGGER AssignAuditReference ON tblBooking
FOR INSERT
AS
DECLARE @.Ref int
--Get the highest reference and add one.
SELECT @.Ref = isnull(max(job_id),0)+1 from tblBooking
--Update the inserted row to have a booking_referecen of the new reference
obtained above.
Update tblBooking
SET Booking_Reference = @.ref
WHERE Booking_ID = INSERTED.Booking_ID
However I'm getting problems with the INSERTED table not being recognised.
I understood that the INSERTED table contained the row that the insert that
started the trigger inserted.
Two questions:
1. Where am I going wrong with my trigger. Have I misunderstood some key
point of using triggers.
2. Is this the right approach to achieve what I am after? Are there any
better approaches...Hi Chris,
You have to mention the Inserted Table in your Update Query
Update tblBooking
SET tblBooking.Booking_Reference = @.Ref
FROM tblBooking
INNER JOIN INSERTED
ON (tblBooking.Booking_ID= INSERTED.Booking_ID)
Because of the lack between getting the @.Ref-Value and writing it in the
table i would prefer an inline Query and Update
Update tblBooking
SET tblBooking.Booking_Reference = NewJobIdTable.NewJobId
FROM tblBooking,
(
Select ISNULL(MAX(job_id),0)+1 AS NewJobId from tblBooking
) NewJobIdTable
INNER JOIN INSERTED
ON (tblBooking.Booking_ID= INSERTED.Booking_ID)
HTH, Jens Smeyer.
http://www.sqlserver2005.de
--
"Chris Strug" <hotmail@.solace1884.com> schrieb im Newsbeitrag
news:eBcd9GOQFHA.1392@.TK2MSFTNGP10.phx.gbl...
> Hi,
> This is kind of following on from my last couple of posts regarding
> Identity
> columns and so on.
> Basically, I want to ensure that for a particular table, every row has a
> numeric reference. This reference must be unique and gapless. Ideally it
> should order in the sequence of the records being inserted however this
> isn't an absolute requirement.
> From my (limited) understanding of SQLS, I think I can achieve this with a
> FOR INSERT Trigger - in that the trigger is fired every time a row is
> inserted and the trigger is the same transaction as the initial insert
> hence
> I avoid any concurrency issues.
> However I'm not completely sure how to achieve this. I think that my
> trigger
> should be along the lines of this...
> CREATE TRIGGER AssignAuditReference ON tblBooking
> FOR INSERT
> AS
> DECLARE @.Ref int
> --Get the highest reference and add one.
> SELECT @.Ref = isnull(max(job_id),0)+1 from tblBooking
> --Update the inserted row to have a booking_referecen of the new
> reference
> obtained above.
> Update tblBooking
> SET Booking_Reference = @.ref
> WHERE Booking_ID = INSERTED.Booking_ID
> However I'm getting problems with the INSERTED table not being recognised.
> I understood that the INSERTED table contained the row that the insert
> that
> started the trigger inserted.
> Two questions:
> 1. Where am I going wrong with my trigger. Have I misunderstood some key
> point of using triggers.
> 2. Is this the right approach to achieve what I am after? Are there any
> better approaches...
>|||Syntactically, your UPDATE statement is missing the FROM clasue:
UPDATE tblBooking
SET Booking_Reference = @.ref
FROM tblBooking , INSERTED
WHERE Booking_ID = INSERTED.Booking_ID
1) This will FAIL if more than one row is inserted - not a good idea
for maintaining an audit trail. 2) I don't see what advantage this has
over the more concise and reliable solution(s) already discussed in
your earlier threads. For example:
INSERT INTO tblBooking (booking_reference, x, y, z, ...)
SELECT COALESCE(MAX(booking_reference),0)+1, 'foo', 'bar', 1234, ...
FROM tblBooking
IMO an incrementing counter is a poor way to maintain an audit trail
anyway. Why not just store the CURRENT_TIMESTAMP on each row and then
preserve the history of changes to rows? This is easy to do in triggers
or in your data access code and doesn't suffer the inevitable and
serious blocking problems that your approach implies.
David Portas
SQL Server MVP
--|||"David Portas" <REMOVE_BEFORE_REPLYING_dportas@.acm.org> wrote in message
news:1113478283.991829.222620@.g14g2000cwa.googlegroups.com...
> Syntactically, your UPDATE statement is missing the FROM clasue:
> UPDATE tblBooking
> SET Booking_Reference = @.ref
> FROM tblBooking , INSERTED
> WHERE Booking_ID = INSERTED.Booking_ID
> 1) This will FAIL if more than one row is inserted - not a good idea
> for maintaining an audit trail. 2) I don't see what advantage this has
> over the more concise and reliable solution(s) already discussed in
> your earlier threads. For example:
> INSERT INTO tblBooking (booking_reference, x, y, z, ...)
> SELECT COALESCE(MAX(booking_reference),0)+1, 'foo', 'bar', 1234, ...
> FROM tblBooking
> IMO an incrementing counter is a poor way to maintain an audit trail
> anyway. Why not just store the CURRENT_TIMESTAMP on each row and then
> preserve the history of changes to rows? This is easy to do in triggers
> or in your data access code and doesn't suffer the inevitable and
> serious blocking problems that your approach implies.
> --
> David Portas
> SQL Server MVP
> --
>
First of all thanks to both David and Jens for their replies.
Apologies for repeating myself, I just want to make sure that I understand
what I'm doing rather than repeating it parrot fashion into my database.
Regarding the trigger, I was under the impression that the trigger would
occur for every new row, I gather that it in fact applies to every INSERT.
Ahh... That makes things clearer.
Regards the actual implementation (TIMESTAMP vs. numeric reference),
unfortunately this is out of my hands. I've been informed that this a is a
non negotiatable requirement. What can you do?
if I may ask one more question, assuming that I did attempt to implement my
apprioach using triggers, could you expand on the blocking problems that you
would expect me to face?
Anyway, thank you once again for taking the time to help me, I do appreciate
it.
Regards
Chris.