just wondering if it is possible to view the changes of an update command on a table after it has happened?
would the transaction log store this kind of information i.e this is the previous/current value this is the new/changed value
not sure if this is possible. the server is ms sql 2008
you can use OUTPUT to insert the changes into another table and/or return a result set of the changes:
set nocount on
DECLARE @Table table (PK int, col1 varchar(5))
DECLARE @SavedPks table (PK int)
INSERT INTO @Table VALUES (1,'g')
INSERT INTO @Table VALUES (2,'g')
INSERT INTO @Table VALUES (3,'g')
INSERT INTO @Table VALUES (4,'g')
INSERT INTO @Table VALUES (5,'x')
INSERT INTO @Table VALUES (6,'x')
set nocount off
UPDATE @Table
SET col1='xyz'
OUTPUT INSERTED.PK INTO @SavedPks --save in a table
OUTPUT 'result set',INSERTED.PK, INSERTED.col1 AS new_col1, DELETED.col1 AS old_Col1 --result set
WHERE col1='g'
select * from @Table
select * from @SavedPks
OUTPUT:
PK new_col1 old_Col1
---------- ----------- -------- --------
result set 1 xyz g
result set 2 xyz g
result set 3 xyz g
result set 4 xyz g
(4 row(s) affected)
PK col1
----------- -----
1 xyz
2 xyz
3 xyz
4 xyz
5 x
6 x
(6 row(s) affected)
PK
-----------
1
2
3
4
The transaction log does contain the information, although decoding it is not trivial - I wrote a blog post with an example of decoding a simple update statement in the transaction log - http://sqlfascination.com/2010/02/21/decoding-a-simple-update-statement-within-the-transaction-log/
However, depending on the update being performed the entries in the log get a lot more complex and there is no documentation available on how to decode them. Reverse engineering them is quite difficult.
I would never recommend that this technique of inspecting the log is used to retrieve data, it is only worth looking into from an internals learning perspective / forensic perspective.
