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SQL Trigger for changed columns?

I have never written a trigger before and now am seeing the light. Is
there a way to write a trigger so that if a user changes any column in
a single row on one table then the trigger will write the value of
this (these) rows to a second table. I don't want the unchanged
columns, just the changed columns (with column names...)
Jul 20 '05
10 39508
On 15 Apr 2004 15:49:24 -0700, Lauren Quantrell wrote:
Thanks for your replies and your help.
I'm trying to keep a record of every change a user makes to any field
on a form.
The underlying recordsource is an SP that refers to a single table.
The user can only modify one row at a time.

Ideally, I would track changes to datetime, int, nvarchar and ntext
This is used entirely as an audit trail, but users would be able to
click a button to view the changes any user had made.

The table holding the audit would need a column for the fieldname
nvarchar(25) , databeforechang e (?), dataafterchange (?), userID int,
dateofchange smalldatetime.

This must be a pretty common need?


Hi Lauren,

The need is pretty common indeed. But you have chosen an approach that
differs from how I normally solve this.

I prefer to make an audit table with the same rows as the actual data
table (but all except the key columns nullable), plus extra rows (not
nullable) for userid, time of change, etc. PK of this table is equal
to the PK of the actual data table plus time of change.

Then, I make the following triggers:
* One trigger for insert, update that enters all "new" information
(from the inserted pseudo-table) into the audit table
* One trigger for delete that enters just the key columns into the
audit table. If all non-key columns in the actual data table are
nullable, you'll also need a "deleted" indication to see the
difference between a deletion and an update that sets all non-key
columns to null.

Advantage of this approach is that you have all information at a
glance, the triggers are easy to set up and will also work when
set-based changes are carried out on the data and it saves some space
if changes usually affect multiple columns at the same time.

Disadvantage is that your approach costs less space if changes usually
affect only one or two columns. That being said, I'd still choose "my"
approach even if it costs more disk space - I value ease of use and
maintainability more than disk space.

(Note - the ntext columns might require special handling, in both
scenario's. In "your" version because a sql_variant datatype can't be
used to hold ntext (if I recall correctly); in "my" version because it
might be worthwile to exclude this column from the audit table if the
text is rarely changed. Besides, I don't know how ntext data is
handled in triggers and if all set-based operations are allowed on
ntext data)

Best, Hugo

(Remove _NO_ and _SPAM_ to get my e-mail address)
Jul 20 '05 #11

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