Question

I have a table called Objects which contains some files, say:

1. User
2. Teacher

There is another table (States) which holds the possible states of these objects, like:

1. Active
2. Idle
3. Teaching
4. Resting
5. Authoring

And there is a third table (junction table) which logs each state change of each object. In this third table (ObjectStates) records are like:

1. 1, 1, DateTime1 (User was active on DateTime1)
2. 2, 5, DateTime2 (Teacher was authoring on DateTime2)

etc.

Now, what I want is a query to get each object, with its latest state (not state history). It’s possible to get this result using cursors, or using Cross Apply command. However, I’d like to know if there is any other way to get the latest states of each object from these three tables? Because cursors are costy.

Answer 1

Using the row_number() windowing function…

select * 
from
(

select objects.*,
       state.state,
       objectstates.changedate,
       row_number() over (partition by object.objectid order by changedate desc) rn
from 
    objects
         inner join
    objectstates
         on objects.id = objectstates.objectid
         inner join
    states 
         on objectstates.stateid = states.stateid
) v
where rn = 1

If you can’t use row_number because you’re on SQL 2000, for example, you can use a join on a max/group by query.

select objects.*,
       state.state,
       objectstates.changedate,
from 
    objects
         inner join
    objectstates
         on objects.id = objectstates.objectid
         inner join
    states 
         on objectstates.stateid = states.stateid
    inner join
         (select objectid, max(changedate) as maxdate from objectstates group by objectid) maxstates
         on objectstates.objectid=maxstates.objectid
         and objectstates.changedate = maxstates.maxdate