Get Date of Change - sql

Question

I have a table containing Dates and Statuses. I wish to get the date that the status changed to the most recent status. Sample data:
DATE STATUS
01/01/2000 P
02/01/2000 A
03/01/2000 C
04/01/2000 A
05/01/2000 A
06/01/2000 A
So in this instance the most recent status is A and it changed to this on 04/01/2000. (The 02/01/2000 row should be ignored in this situation)
Any suggestions for how to go about selecting this row?

Answer 1

At first, I misunderstood the question. You need to get the earliest date of the last status.
You can group sequences of like statuses using a trick -- a difference of row numbers. The difference (in the query below) is constant for sequences that are the same. Then you can use aggregation to get the minimum date and select the latest one:
select mindate
from (select min(date) as mindate
from (select t.*,
row_number() over (order by date) as seqnum1,
row_number() over (partition by status order by date) as seqnum2
from table t
) t
group by status, (seqnum1 - seqnum2)
order by mindate desc
) t
where rownum = 1
EDIT:
In any case, the right way to do this is using lag():
select max(date)
from (select t.*, lag(status) over (order by date) as prev_status
from table t
)
where prev_status <> status or prev_status is null;
Here is the SQL Fiddle.

Answer 2

You can do this using lag or lead. Here I'm using lead, ordering by date descending to find the previous status date (if it's null I'm just supplying the date, which is needed in case there's only one record).
select max(date)
from (
select status, date, nvl(lead(status) over (order by date desc),date) as previous_status
from t
order by date desc
)
where status <> previous_status;

Answer 3

Something like this ought to do the trick:
with sample_data as (select to_date('01/01/2000', 'dd/mm/yyyy') dt, 'P' status from dual union all
select to_date('02/01/2000', 'dd/mm/yyyy') dt, 'A' status from dual union all
select to_date('03/01/2000', 'dd/mm/yyyy') dt, 'C' status from dual union all
select to_date('04/01/2000', 'dd/mm/yyyy') dt, 'A' status from dual union all
select to_date('05/01/2000', 'dd/mm/yyyy') dt, 'A' status from dual union all
select to_date('06/01/2000', 'dd/mm/yyyy') dt, 'A' status from dual),
results1 as (select dt,
status,
row_number() over (order by dt) - row_number() over (partition by status order by dt) grp
from sample_data),
results2 as (select status, min(dt) min_dt, grp, max(min(dt)) over () max_min_dt
from results1
group by status, grp)
select status, min_dt
from results2
where min_dt = max_min_dt;
STATUS MIN_DT
------ ----------
A 04/01/2000