SQL Trigger to record UPDATE into Audit Table - sql

Question

I have a table with columns businessname, sortcode and accountnumber all populated, name, nationality and DOB all currently unpopulated. I need to create a trigger to shoot every update to an audit table when any of the null fields are updated so if I change just the name I'll get a timestamp, userid, the field changed, the old value and the new value.. If I changed all 3 null fields I'd like to send 3 rows to the audit table.
Can someone give me a pointer on the logic of this please?
In a very rudimentary format for testing I've got
CREATE TRIGGER RM_UPDATE_TRIGGER ON RM_BASE
ON UPDATE
AS
INSERT INTO RM_AUDITLOG
SELECT CURRENT_TIMESTAMP, SRT_CD
FROM RM_BASE
but this is sending all the current rows across after an UPDATE to any of them, I only want the row that has been affected. I'm not sure if I should be building more tables to join together to get the final answer or using the INSERT/DELETE tables.. I've seen an audit table in this format in previous roles so I know it works but can't figure it out!
Thanks

Answer 1

Yes, you need to use the INSERTED and/or DELETED pseudo-tables as those contain only the rows that have been modified. As in:
CREATE TRIGGER RM_UPDATE_TRIGGER ON RM_BASE
ON UPDATE
AS
INSERT INTO RM_AUDITLOG
SELECT CURRENT_TIMESTAMP, SRT_CD
FROM INSERTED
The INSERTED table has the "new" or "current" version of each row while the DELETED table has the "old" version that has been replaced via the UPDATE operation. This is the case for all versions of SQL Server, at least going back as far as SQL Server 2000.
In order to track the change itself (both "old" and "new" values), then you need to JOIN those two pseudo-tables, as in:
INSERT INTO RM_AUDITLOG
SELECT CURRENT_TIMESTAMP, ins.SRT_CD AS [SRT_CD_new], del.SRT_CD AS [SRT_CD_old]
FROM INSERTED ins
INNER JOIN DELETED del
ON del.PKfield = ins.PKfield
This is the basic operation for capturing changes (unless, of course, you use Change Data Capture) in a DML trigger.
If you want to unpivot this data such that each set of "old" and "new" columns becomes a row, that should be easily adaptable from the above. In that case, you could also add WHERE ISNULL(ins.column, '~~~~') <> ISNULL(del.column, '~~~~') COLLATE Latin1_General_BIN to avoid capturing fields that have not changed. The COLLATE ensures case-sensitive / accent-sensitive / etc comparisons.
Of course, unpivoting makes it really hard to reconstruct the entire row as you are then required to keep the full history forever. You would need to start with the base values for all fields and apply the changes incrementally. The typical audit scenario is to just to capture the row that has fields for both old and new for each source field (like I have already shown). If your audit table looks more like:
PKfield, DateModified, businessname_old, businessname_new, sortcode_old, sortcode_new
then you can write a query to identify which fields actually changed by comparing each set (given that more than 1 field can change in the same UPDATE operation), something like:
SELECT PKfield,
DateModified,
CASE
WHEN ISNULL(businessname_old, '~~~~') <> ISNULL(businessname_new, '~~~~')
COLLATE Latin1_General_BIN THEN 'BusinessName ' ELSE ''
END +
CASE
WHEN ISNULL(sortcode_old, '~~~~') <> ISNULL(sortcode_new, '~~~~')
COLLATE Latin1_General_BIN THEN 'SortCode ' ELSE ''
END AS [FieldsChanged]
FROM AuditTable
ORDER BY DateModified DESC;
BUT, if you really want to unpivot the data to have one row per actual changed field, then the following structure should work:
;WITH ins AS
(
SELECT PKfield, FieldName, Value
FROM (
SELECT PKfield, businessname, sortcode, accountnumber, name,
nationality, DOB
FROM INSERTED
) cols
UNPIVOT (Value FOR FieldName IN
(businessname, sortcode, accountnumber, name, nationality, DOB)
) colvals
), del AS
(
SELECT PKfield, FieldName, Value
FROM (
SELECT PKfield, businessname, sortcode, accountnumber, name,
nationality, DOB
FROM DELETED
) cols
UNPIVOT (Value FOR FieldName IN
(businessname, sortcode, accountnumber, name, nationality, DOB)
) colvals
)
INSERT INTO AuditTable (PKfield, DateModified, FieldName, OldValue, NewValue)
SELECT ins.PKfield, CURRENT_TIMESTAMP, ins.FieldName, del.Value, ins.Value
FROM ins
INNER JOIN del
ON del.PKfield = ins.PKfield
AND del.FieldName = ins.FieldName
WHERE ISNULL(del.Value, '~~~~') <>
ISNULL(ins.Value, '~~~~') COLLATE Latin1_General_BIN;
You might need to add a CONVERT(VARCHAR(1000), field) to the WHERE condition if DOB is a DATE or DATETIME field, or if SRT_CD is an INT or other type of number field:
WHERE ISNULL(CONVERT(VARCHAR(1000), del.Value), '~~~~') <>
ISNULL(CONVERT(VARCHAR(1000), ins.Value), '~~~~')
COLLATE Latin1_General_BIN;