Sql query to regroup audit information

Question

I have a SQL Server database where there is an history table containing a record each time the said record, in another table, changes.

The source data table:

Id	DataA	DataB	DataC
1	Truc	Much	Bazar

The history table:

DataId	Time	Username	DataA	DataB	DataC
1	2023-01-01 12:00:00	User1	Truc	Much	Bazar
1	2023-01-01 06:00:00	User1	Truc	Much	Bazar2
1	2023-01-01 00:00:00	User1	Truc	Much2	Bazar2

Meaning the record 1 of the data table has been updated 3 times by user "User1", He changed "Much2" to "Much" and "Bazar2" to "Bazar". The last record (first line) being the current state.

I would like to make a query that shows the audit in a way we can see which column has been updated, with the old value, the new one, and by who and when

In this example, it will be like this:

DataId	Time	Username	Column	Old value	New value
1	2023-01-01 12:00:00	User1	DataC	Bazar2	Bazar
2	2023-01-01 06:00:00	User1	DataB	Much2	Much

I'm clearly not an expert in SQL, and I don't really know how to handle this. Any help will be appreciated

Answer 1

You can unpivot your data columns to rows, then use window functions to identify values that changed:

select dataid, time, username, col, lag_val as old_value, val as new_value
from (
    select t.*, v.*,
        lag(val, 1, val) over(partition by dataid, col order by time) lag_val
    from mytable t
    cross apply ( values ('DataA', DataA), ('DataB', DataB), ('DataC', DataC) ) v(col, val)
) t
where val <> lag_val
order by dataid, time

dataid	time	username	col	old_value	new_value
1	2023-01-01 06:00:00.000	User1	DataB	Much2	Much
1	2023-01-01 12:00:00.000	User1	DataC	Bazar2	Bazar

fiddle