Select only changes to a value from an audit log table

Question

In MS SQL Server 2008 R2, I have a table Foo, and for every insert and update on Foo, I also insert into FooAuditLog the date, user, the PK FooId and a few of the other columns of Foo, including one numeric value, call it cxp.

I now need to retrieve the history of changes to cxp over time for a given FooId. But it is possible to save a Foo without changing cxp, and I need to ignore those values.

For example, if the Audit Log entries for a specific Foo (ie select date, user, cxp from FooAuditLog where fooId=17) look like this:

Date      User     Cxp
-------------------------
10/26     Fred     42
10/28     George   38
11/7      Tom      38
11/9      Fred     38
11/12     Joe      33
11/14     Tom      33
11/18     George   38

Then I need to modify the query to return only:

Date      User     Cxp
-----------------------------
10/26     Fred     42
10/28     George   38
11/12     Joe      33
11/18     George   38

And ignore the entries on 11/7, 11/9, and 11/14. I had considered a select distinct (cpx) ... group by date but I do need to capture the entry where the value changes back to a previous value.

Answer 1

You need to get the previous value. This version uses MySQL syntax with a correlated subquery to get the result:

select t.*
from (select t.*,
             (select cxp from t t2 where t2.date < t.date order by date desc limit1
             ) as prevcxp
      from t
     ) t
where prevcxp is NULL or prevcxp <> cxp

In other databases, you might use lag() instead of the subquery, the limit might be replaced by a top or even fetch first 1 row.