Counting current items by month - sql

Question

I'm trying to build a monthly tally of active equipment, grouped by service area from a database log table. I think I'm 90% of the way there; I have a list of months, along with the total number of items that existed, and grouped by region.
However, I also need to know the state of each item as they were on the first of each month, and this is the part I'm stuck on. For instance, Item 1 is in region A in January, but moves to Region B in February. Item 2 is marked as 'inactive' in February, so shouldn't be counted. My existing query will always count item 1 in region A, and item 2 as 'active'.
I can correctly show that Item 3 is deleted in March, and Item 4 doesn't show up until the April count. I realize that I'm getting the first values because my query is specifying the min date, I'm just not sure how I need to change it to get what I want.
I think I'm looking for a way to group by Max(OperationDate) for each Month.
The Table looks like this:
| EQUIPID | EQUIPNAME | EQUIPACTIVE | DISTRICT | REGION | OPERATIONDATE | OPERATION |
|---------|-----------|-------------|----------|--------|----------------------|-----------|
| 1 | Item 1 | 1 | 1 | A | 2015-01-01T00:00:00Z | INS |
| 2 | Item 2 | 1 | 1 | A | 2015-01-01T00:00:00Z | INS |
| 3 | Item 3 | 1 | 1 | A | 2015-01-01T00:00:00Z | INS |
| 2 | Item 2 | 0 | 1 | A | 2015-02-10T00:00:00Z | UPD |
| 1 | Item 1 | 1 | 1 | B | 2015-02-15T00:00:00Z | UPD |
| 3 | (null) | (null) | (null) | (null) | 2015-02-21T00:00:00Z | DEL |
| 1 | Item 1 | 1 | 1 | A | 2015-03-01T00:00:00Z | UPD |
| 4 | Item 4 | 1 | 1 | B | 2015-03-10T00:00:00Z | INS |
There is also a subtable that holds attributes that I care about. It's structure is similar. Unfortunately, due to previous design decisions, there is no correlation to operations between the two tables. Any joins will need to be done using the EquipmentID, and have the overlapping states matched up for each date.
Current query:
--cte to build date list
WITH calendar (dt) AS
(SELECT &fromdate from dual
UNION ALL
SELECT Add_Months(dt,1)
FROM calendar
WHERE dt < &todate)
SELECT dt, a.district, a.region, count(*)
FROM
(SELECT EQUIPID, DISTRICT, REGION, OPERATION, MIN(OPERATIONDATE ) AS FirstOp, deleted.deldate
FROM Equipment_Log
LEFT JOIN
(SELECT EQUIPID,MAX(OPERATIONDATE) as DelDate
FROM Equipment_Log
WHERE OPERATION = 'DEL'
GROUP BY EQUIPID
) Deleted
ON Equipment_Log.EQUIPID = Deleted.EQUIPID
WHERE OPERATION <> 'DEL' --AND additional unimportant filters
GROUP BY EQUIPID,DISTRICT, REGION , OPERATION, deldate
) a
INNER JOIN calendar
ON (calendar.dt >= FirstOp AND calendar.dt < deldate)
OR (calendar.dt >= FirstOp AND deldate is null)
LEFT JOIN
( SELECT EQUIPID, MAX(OPERATIONDATE) as latestop
FROM SpecialEquip_Table_Log
--where SpecialEquip filters
group by EQUIPID
) SpecialEquip
ON a.EQUIPID = SpecialEquip.EQUIPID and calendar.dt >= SpecialEquip.latestop
GROUP BY dt, district, region
ORDER BY dt, district, region

Answer 1

Take only last operation for each id. This is what row_number() and where rn = 1 do.
We have calendar and data. Make partitioned join.
I assumed that you need to fill values for months where entries for id are missing. So nvl(lag() ignore nulls) are needed, because if something appeared in January it still exists in Feb, March and we need district, region values from last not empty row.
Now you have everything to make count. That part where you mentioned SpecialEquip_Table_Log is up to you, because you left-joined this table and not used it later, so what is it for? Join if you need it, you have id.
db<>fiddle
with
calendar(mth) as (
select date '2015-01-01' from dual union all
select add_months(mth, 1) from calendar where mth < date '2015-05-01'),
data as (
select id, dis, reg, dt, op, act
from (
select equipid id, district dis, region reg,
to_char(operationdate, 'yyyy-mm') dt,
row_number()
over (partition by equipid, trunc(operationdate, 'month')
order by operationdate desc) rn,
operation op, nvl(equipactive, 0) act
from t)
where rn = 1 )
select mth, dis, reg, sum(act) cnt
from (
select id, mth,
nvl(dis, lag(dis) ignore nulls over (partition by id order by mth)) dis,
nvl(reg, lag(reg) ignore nulls over (partition by id order by mth)) reg,
nvl(act, lag(act) ignore nulls over (partition by id order by mth)) act
from calendar
left join data partition by (id) on dt = to_char(mth, 'yyyy-mm') )
group by mth, dis, reg
having sum(act) > 0
order by mth, dis, reg
It may seem complicated, so please run subqueries separately at first to see what is going on. And test :) Hope this helps.