Question

I got a table like this

Measurements

id    object_id   generated_at (v)    
110   4           2012-11-13 10:29:00   
109   4           2012-11-13 10:28:00   
108   4           2012-11-13 10:27:00
107   3           2012-11-13 10:26:00
106   3           2012-11-13 10:25:00
105   4           2012-11-13 10:24:00
104   4           2012-11-13 10:23:00
103   1           2012-11-13 10:22:00
102   1           2012-11-13 10:21:00
...

ordered by generated_at column.

My question: is there any ability to select

id    object_id   generated_at (v)    
110   4           2012-11-13 10:29:00   
109   4           2012-11-13 10:28:00   
108   4           2012-11-13 10:27:00

last group(ordered by generated_at) of measurements with same object_id ?

It could be example in rails for Measurement model or just SQL query, thanks for any help!

Answer 1

You original question clearly wanted the objects in the most recent groups, not all objects. For this, you need to calculate both the max object id and the maximum generated time for other objects:

select *
from t cross join
     (select max(object_id) maxobject_id from t) maxt
where object_id = maxt.maxobject_id and
      generated_at > (select max(generated_at) from t where object_id <> maxt.maxobject_id)