Help with aggregate function max query

Question

I have a large database which stores daily oil life, odo readings from thousands of vehicles being tested but only once a month.
I am trying to grab data from one month where only the EOL_READ = 0 and put all of those values into the previous month's query where EOL_READ = anything.
Here is the original query which grabs everything
(select distinct vdh.STID, vdh.CASE_SAK,
max(to_number(decode(COMMAND_ID,'EOL_READ',COMMAND_RESULT,-100000))) EOL_READ,
max(to_number(decode(COMMAND_ID,'ODO_READ',COMMAND_RESULT,-100000))) ODO_READ,
max(to_number(decode(COMMAND_ID,'OIL_LIFE_PREDICTION',COMMAND_RESULT,-100000))) OIL_LIFE_PREDICTION
from veh_diag_history vdh, c2pt_data_history cdh
where vdh.veh_diag_history_sak = cdh.veh_diag_history_sak
and cdh.COMMAND_ID in ('EOL_READ','ODO_READ','OIL_LIFE_PREDICTION')
and vdh.LOGICAL_TRIGGER_SAK = 3
and cdh.CREATED_TIMESTAMP =vdh.CREATED_TIMESTAMP
and cdh.CREATED_TIMESTAMP >= to_date('03/01/2007 12:00:00 AM','mm/dd/yyyy HH:MI:SS AM')
and cdh.CREATED_TIMESTAMP < to_date('04/01/2007 12:00:00 AM','mm/dd/yyyy HH:MI:SS AM')
group by vdh.STID, vdh.case_sak
having count(distinct command_id) = 3
order by OIL_LIFE_PREDICTION)
which gives 5 columns:
STID, CASE_SAK, EOL_READ, ODO_READ, and OIL_LIFE_PREDICTION
and gives me about 80000 rows returned for the above query
I only want one reading per month, but sometimes I get two.
STID is the unique identifier for a vehicle.
I tried this query:
I tried this query which nests one request within the other and SQL times out every time:
select distinct vdh.STID, vdh.CASE_SAK,
max(to_number(decode(COMMAND_ID,'EOL_READ',COMMAND_RESULT,-100000))) EOL_READ,
max(to_number(decode(COMMAND_ID,'ODO_READ',COMMAND_RESULT,-100000))) ODO_READ,
max(to_number(decode(COMMAND_ID,'OIL_LIFE_PREDICTION',COMMAND_RESULT,-100000))) OIL_LIFE_PREDICTION
from veh_diag_history vdh, c2pt_data_history cdh
where vdh.veh_diag_history_sak = cdh.veh_diag_history_sak
and cdh.COMMAND_ID in ('EOL_READ','ODO_READ','OIL_LIFE_PREDICTION')
and vdh.LOGICAL_TRIGGER_SAK = 3
and cdh.CREATED_TIMESTAMP =vdh.CREATED_TIMESTAMP
and cdh.CREATED_TIMESTAMP >= to_date('02/01/2007 12:00:00 AM','mm/dd/yyyy HH:MI:SS AM')
and cdh.CREATED_TIMESTAMP < to_date('03/01/2007 12:00:00 AM','mm/dd/yyyy HH:MI:SS AM')
group by vdh.STID, vdh.case_sak
having count(distinct command_id) = 3
and vdh.stid in (select distinct vdh.STID, vdh.CASE_SAK,
max(to_number(decode(COMMAND_ID,'EOL_READ',COMMAND_RESULT,-100000))) EOL_READ,
max(to_number(decode(COMMAND_ID,'ODO_READ',COMMAND_RESULT,-100000))) ODO_READ,
max(to_number(decode(COMMAND_ID,'OIL_LIFE_PREDICTION',COMMAND_RESULT,-100000))) OIL_LIFE_PREDICTION
from veh_diag_history vdh, c2pt_data_history cdh
where vdh.veh_diag_history_sak = cdh.veh_diag_history_sak
and cdh.COMMAND_ID in ('EOL_READ','ODO_READ','OIL_LIFE_PREDICTION')
and vdh.LOGICAL_TRIGGER_SAK = 3
and cdh.CREATED_TIMESTAMP =vdh.CREATED_TIMESTAMP
and cdh.CREATED_TIMESTAMP >= to_date('03/01/2007 12:00:00 AM','mm/dd/yyyy HH:MI:SS AM')
and cdh.CREATED_TIMESTAMP < to_date('04/01/2007 12:00:00 AM','mm/dd/yyyy HH:MI:SS AM')
group by vdh.STID, vdh.case_sak
having count(distinct command_id) = 3
and (max(to_number(decode(COMMAND_ID,'EOL_READ',COMMAND_RESULT,-100000)))) = 0)
order by OIL_LIFE_PREDICTION
so in summary I am trying to select values from the previous month only from those stids where this month's value for EOL_READ = 0
Any ideas.....please help.

Answer 1

You can use row_number() within each stid and each month to determine the first read of each month. Then you can use lag() to get the previous month's reading of the current month's reading is zero.
with all_data as (
select stid,
       case_sak,
       eol_read,
       timestamp,
       row_number() over (partition by stid, trunc(timestamp,'mm') order by timestamp) AS rn
  from veh_diag_history
select stid,
       case_sak,
       case
         when eol_read = 0
           then lag(eol_read) over (partition by stid order by timestamp)
         else eol_read
       end as eol_read
  from all_data
where rn = 1;