Passing info on who deleted record onto a Delete trigger

Question

In setting up an audit trail I have no problem tracking who is updating or inserting records in a table, however, tracking who deletes records seems more problematic.

I can track Inserts/Updates by including in the Insert/Update the field "UpdatedBy". This allows the INSERT/UPDATE trigger to have access to the field "UpdatedBy" via inserted.UpdatedBy. However, with the Delete trigger no data is inserted/updated. Is there a way to pass information onto the Delete trigger such that it could know who deleted the record?

Here is an Insert/Update trigger

ALTER TRIGGER [dbo].[trg_MyTable_InsertUpdate] 
ON [dbo].[MyTable]
FOR INSERT, UPDATE
AS  

INSERT INTO AuditTable (IdOfRecordedAffected, UserWhoMadeChanges) 
VALUES (inserted.ID, inserted.LastUpdatedBy)
FROM inserted 

Using SQL Server 2012

Answer 1

Is there a way to pass information onto the Delete trigger such that it could know who deleted the record?

Yes: by using a very cool (and under utilized feature) called CONTEXT_INFO. It is essentially session memory that exists in all scopes and is not bound by transactions. It can be used to pass info (any info--well, any that fits into the limited space) to triggers as well as back and forth between sub-proc / EXEC calls. And I have used it before for this exact same situation.

• Context info is a VARBINARY(128)

• Set via: SET CONTEXT_INFO

• Get via: CONTEXT_INFO()

Test with the following to see how it works. Notice that I am converting to CHAR(128) before the CONVERT(VARBINARY(128), ... This is to force blank-padding to make it easier to convert back to VARCHAR when getting it out of CONTEXT_INFO() since VARBINARY(128) is right-padded with 0x00s.

SELECT CONTEXT_INFO();
-- Initially = NULL

DECLARE @EncodedUser VARBINARY(128);
SET @EncodedUser = CONVERT(VARBINARY(128),
                            CONVERT(CHAR(128), 'I deleted ALL your records! HA HA!')
                          );
SET CONTEXT_INFO @EncodedUser;

SELECT CONTEXT_INFO() AS [RawContextInfo],
       RTRIM(CONVERT(VARCHAR(128), CONTEXT_INFO())) AS [DecodedUser];

Results:

0x492064656C6574656420414C4C20796F7572207265636F7264732120484120484121202020202020...
I deleted ALL your records! HA HA!

PUTTING IT ALL TOGETHER:

1. The app should call a "Delete" stored procedure that passes in the UserName (or whatever) that is deleting the record. I assume this is already the model being used since it sounds like you are already tracking Insert and Update operations.

2. The "Delete" stored procedure does:

   DECLARE @EncodedUser VARBINARY(128);
   SET @EncodedUser = CONVERT(VARBINARY(128),
                               CONVERT(CHAR(128), @UserName)
                             );
   SET CONTEXT_INFO @EncodedUser;
   
   -- DELETE STUFF HERE
   

3. The audit trigger does:

   -- Set the INT value in LEFT (currently 50) to the max size of [UserWhoMadeChanges]
   INSERT INTO AuditTable (IdOfRecordedAffected, UserWhoMadeChanges) 
      SELECT del.ID, COALESCE(
                        LEFT(RTRIM(CONVERT(VARCHAR(128), CONTEXT_INFO())), 50),
                        '<unknown>')
      FROM DELETED del;
   

4. Please note that, as @SeanGallardy pointed out in a comment, due to other procedures and/or ad hoc queries deleting records from this table, it is possible that either:

   • CONTEXT_INFO has not been set and is still NULL:

     For this reason I have updated the above INSERT INTO AuditTable to use a COALESCE to default the value. Or, if you don't want a default and require a name, then you could do something similar to:

     DECLARE @UserName VARCHAR(50); -- set to the size of AuditTable.[UserWhoMadeChanges]
     SET @UserName = LEFT(RTRIM(CONVERT(VARCHAR(128), CONTEXT_INFO())), 50);
     
     IF (@UserName IS NULL)
     BEGIN
        ROLLBACK TRAN; -- cancel the DELETE operation
        RAISERROR('Please set UserName via "SET CONTEXT_INFO.." and try again.', 16 ,1);
     END;
     
     -- use @UserName in the INSERT...SELECT
     

   • CONTEXT_INFO has been set to a value that is not a valid UserName, and hence might exceed the size of the AuditTable.[UserWhoMadeChanges] field:

     For this reason I added a LEFT function to ensure that whatever is grabbed out of CONTEXT_INFO will not break the INSERT. As noted in the code, you just need to set the 50 to the actual size of the UserWhoMadeChanges field.

---

UPDATE FOR SQL SERVER 2016 AND NEWER

SQL Server 2016 added an improved version of this per-session memory: Session Context. The new Session Context is essentially a hash table of Key-Value pairs with the "Key" being of type sysname (i.e. NVARCHAR(128) ) and the "Value" being SQL_VARIANT. Meaning:

1. There is now a separation of values so less likely to conflict with other uses
2. You can store various types, no longer needing to worry about the odd behavior when getting the value back out via CONTEXT_INFO() (for details, please see my post: Why Doesn’t CONTEXT_INFO() Return the Exact Value Set by SET CONTEXT_INFO?)
3. You get a lot more space: 8000 bytes max per "Value", up to 256kb total across all keys (compared to the 128 bytes max of CONTEXT_INFO)

For details, please see the following documentation pages:

• sp_set_session_context
• SESSION_CONTEXT

Answer 2

You can't that way, unless you are looking to record the SQL server user ID rather than an application level one.

You can do a soft delete by having a column called DeletedBy and setting that as needed, then your update trigger can do the real delete (or archive the record, I generally avoid hard deletes where possible and legal) as well as updating your audit trail. To force deletes to be done that way define an on delete trigger that raises an error. If you don't want to add a column to your physical table, you could define a view that adds the column and define instead of triggers to handle updating the base table, but that may be overkill.

Answer 3

Is there a way to pass information onto the Delete trigger such that it could know who deleted the record?

Yes, apparently there are two ways ;-). If there are any reservations about using CONTEXT_INFO as I have suggested in my other answer here, I just thought of another way that has a cleaner functional separation from other code / processes: use a local temporary table.

The temp table name should include the table name being deleted from as that will help keep it separate from any other code that might happen to run in the same session. Something along the lines of:
#<TableName>DeleteAudit

One benefit to a local temp table over CONTEXT_INFO is that if someone in another proc--that is somehow call from this particular "Delete" proc--just happens to incorrect use the same temp table name, the subprocess will a) create a new local temp table of the requested name that will be separate from this initial temp table (even though it has the same name), and b) any DML statements against the new local temp table in the sub-process won't affect any data in the local temp table created here in the parent process, hence no overwriting of data. Of course, if a subprocess issues a DML statement against this temp table name without first issuing a CREATE TABLE of that same name, then those DML statements will affect the data in this table. BUT, at this point we are getting really edge-casey here, even more so than with the probability of overlapping uses of CONTEXT_INFO (yes, I know it has happened, which is why I say "edge-case" rather than "it'll never happen").

1. The app should call a "Delete" stored procedure that passes in the UserName (or whatever) that is deleting the record. I assume this is already the model being used since it sounds like you are already tracking Insert and Update operations.

2. The "Delete" stored procedure does:

   CREATE TABLE #MyTableDeleteAudit (UserName VARCHAR(50));
   INSERT INTO #MyTableDeleteAudit (UserName) VALUES (@UserName);
   
   -- DELETE STUFF HERE
   

3. The audit trigger does:

   -- Set the datatype and length to be the same as the [UserWhoMadeChanges] field
   DECLARE @UserName VARCHAR(50);
   IF (OBJECT_ID(N'tempdb..#TriggerTestDeleteAudit') IS NOT NULL)
   BEGIN
      SELECT @UserName = UserName
      FROM #TriggerTestDeleteAudit;
   END;
   
   -- catch the following conditions: missing table, no rows in table, or empty row
   IF (@UserName IS NULL OR @UserName NOT LIKE '%[a-z]%')
   BEGIN
     /* -- uncomment if undefined UserName == badness
      ROLLBACK TRAN; -- cancel the DELETE operation
      RAISERROR('Please set UserName via #TriggerTestDeleteAudit and try again.', 16 ,1);
      RETURN; -- exit
     */
     /* -- uncomment if undefined UserName gets default value
      SET @UserName = '<unknown>';
     */
   END;
   
   INSERT INTO AuditTable (IdOfRecordedAffected, UserWhoMadeChanges) 
      SELECT del.ID, @UserName
      FROM DELETED del;
   

   I have tested this code in a trigger and it works as expected.