Check for changes to an SQL Server table?

Question

How can I monitor an SQL Server database for changes to a table without using triggers or modifying the structure of the database in any way? My preferred programming enviro

Answer 1

Take a look at the CHECKSUM command:

SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM sample_table WITH (NOLOCK);

That will return the same number each time it's run as long as the table contents haven't changed. See my post on this for more information:

CHECKSUM

Here's how I used it to rebuild cache dependencies when tables changed:
ASP.NET 1.1 database cache dependency (without triggers)

Answer 2

Have a DTS job (or a job that is started by a windows service) that runs at a given interval. Each time it is run, it gets information about the given table by using the system INFORMATION_SCHEMA tables, and records this data in the data repository. Compare the data returned regarding the structure of the table with the data returned the previous time. If it is different, then you know that the structure has changed.

Example query to return information regarding all of the columns in table ABC (ideally listing out just the columns from the INFORMATION_SCHEMA table that you want, instead of using *select ** like I do here):

select * from INFORMATION_SCHEMA.COLUMNS where TABLE_NAME = 'ABC'

You would monitor different columns and INFORMATION_SCHEMA views depending on how exactly you define "changes to a table".

Answer 3

How often do you need to check for changes and how large (in terms of row size) are the tables in the database? If you use the CHECKSUM_AGG(BINARY_CHECKSUM(*)) method suggested by John, it will scan every row of the specified table. The NOLOCK hint helps, but on a large database, you are still hitting every row. You will also need to store the checksum for every row so that you tell one has changed.

Have you considered going at this from a different angle? If you do not want to modify the schema to add triggers, (which makes a sense, it's not your database), have you considered working with the application vendor that does make the database?

They could implement an API that provides a mechanism for notifying accessory apps that data has changed. It could be as simple as writing to a notification table that lists what table and which row were modified. That could be implemented through triggers or application code. From your side, ti wouldn't matter, your only concern would be scanning the notification table on a periodic basis. The performance hit on the database would be far less than scanning every row for changes.

The hard part would be convincing the application vendor to implement this feature. Since this can be handles entirely through SQL via triggers, you could do the bulk of the work for them by writing and testing the triggers and then bringing the code to the application vendor. By having the vendor support the triggers, it prevent the situation where your adding a trigger inadvertently replaces a trigger supplied by the vendor.

Answer 4

Why don't you want to use triggers? They are a good thing if you use them correctly. If you use them as a way to enforce referential integrity that is when they go from good to bad. But if you use them for monitoring, they are not really considered taboo.

Answer 5

Check the last commit date. Every database has a history of when each commit is made. I believe its a standard of ACID compliance.

Answer 6

Unfortunately CHECKSUM does not always work properly to detect changes.

It is only a primitive checksum and no cyclic redundancy check (CRC) calculation.

Therefore you can't use it to detect all changes, e. g. symmetrical changes result in the same CHECKSUM!

E. g. the solution with CHECKSUM_AGG(BINARY_CHECKSUM(*)) will always deliver 0 for all 3 tables with different content:

SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM 
(
  SELECT 1 as numA, 1 as numB
  UNION ALL
  SELECT 1 as numA, 1 as numB
)  q
-- delivers 0!


SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM 
(
  SELECT 1 as numA, 2 as numB
  UNION ALL
  SELECT 1 as numA, 2 as numB
)  q
-- delivers 0!

SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM 
(
  SELECT 0 as numA, 0 as numB
  UNION ALL
  SELECT 0 as numA, 0 as numB
)  q
-- delivers 0!