Find minimum value in groups of rows - sql

Question

In the SQL space (specifically T-SQL, SQL Server 2008), given this list of values:
Status Date
------ -----------------------
ACT 2012-01-07 11:51:06.060
ACT 2012-01-07 11:51:07.920
ACT 2012-01-08 04:13:29.140
NOS 2012-01-09 04:29:16.873
ACT 2012-01-21 12:39:37.607 <-- THIS
ACT 2012-01-21 12:40:03.840
ACT 2012-05-02 16:27:17.370
GRAD 2012-05-19 13:30:02.503
GRAD 2013-09-03 22:58:48.750
Generated from this query:
SELECT Status, Date
FROM Account_History
WHERE AccountNumber = '1234'
ORDER BY Date
The status for this particular object started at ACT, then changed to NOS, then back to ACT, then to GRAD.
What is the best way to get the minimum date from the latest "group" of records where Status = 'ACT'?

Answer 1

Here is a query that does this, by identifying the groups where the student statuses are the same and then using simple aggregation:
select top 1 StudentStatus, min(WhenLastChanged) as WhenLastChanged
from (SELECT StudentStatus, WhenLastChanged,
(row_number() over (order by "date") -
row_number() over (partition by studentstatus order by "date)
) as grp
FROM Account_History
WHERE AccountNumber = '1234'
) t
where StudentStatus = 'ACT'
group by StudentStatus, grp
order by WhenLastChanged desc;
The row_number() function assigns sequential numbers within groups of rows based on the date. For your data, the two row_numbers() and their difference is:
Status Date
------ -----------------------
ACT 2012-01-07 11:51:06.060 1 1 0
ACT 2012-01-07 11:51:07.920 2 2 0
ACT 2012-01-08 04:13:29.140 3 3 0
NOS 2012-01-09 04:29:16.873 4 1 3
ACT 2012-01-21 12:39:37.607 5 4 1
ACT 2012-01-21 12:40:03.840 6 5 1
ACT 2012-05-02 16:27:17.370 7 6 1
GRAD 2012-05-19 13:30:02.503 8 1 7
GRAD 2013-09-03 22:58:48.750 9 2 7
Notice the last row is constant for rows that have the same status.
The aggregation brings these together and chooses the latest (top 1 . . . order by date desc) of the first dates (min(date)).
EDIT:
The query is easy to tweak for multiple account numbers. I probably should have written that way to begin with, except the final selection is trickier. The results from this has the date for each status and account:
select StudentStatus, min(WhenLastChanged) as WhenLastChanged
from (SELECT StudentStatus, WhenLastChanged, AccountNumber
(row_number() over (partition by AccountNumber order by WhenLastChanged) -
row_number() over (partition by AccountNumber, studentstatus order by WhenLastChanged)
) as grp
FROM Account_History
) t
where StudentStatus = 'ACT'
group by AccountNumber, StudentStatus, grp
order by WhenLastChanged desc;
But you can't get the last one per account quite so easily. Another level of subqueries:
select AccountNumber, StudentStatus, WhenLastChanged
from (select AccountNumber, StudentStatus, min(WhenLastChanged) as WhenLastChanged,
row_number() over (partition by AccountNumber, StudentStatus order by min(WhenLastChanged) desc
) as seqnum
from (SELECT AccountNumber, StudentStatus, WhenLastChanged,
(row_number() over (partition by AccountNumber order by WhenLastChanged) -
row_number() over (partition by AccountNumber, studentstatus order by WhenLastChanged)
) as grp
FROM Account_History
) t
where StudentStatus = 'ACT'
group by AccountNumber, StudentStatus, grp
) t
where seqnum = 1;
This uses aggregation along with the window function row_number(). This is assigning sequential numbers to the groups (after aggregation), with the last date for each account getting a value of 1 (order by min(WhenLastChanged) desc). The outermost select then just chooses that row for each account.

Answer 2

SELECT [Status], MIN([Date])
FROM Table_Name
WHERE [Status] = (SELECT [Status]
FROM Table_Name
WHERE [Date] = (SELECT MAX([Date])
FROM Table_Name)
)
GROUP BY [Status]
Try here Sql Fiddle

Answer 3

Hogan: basically, yes. I just want to know the date/time when the
account was last changed to ACT. The records after the point above
marked THIS are just extra.
Instead of just looking for act we can look for first time status changes and select act (and max) from that.
so... every time a status changes:
with rownumb as
(
select *, row_number() OVER (order by date asc) as rn
)
select status, date
from rownumb A
join rownumb B on A.rn = B.rn-1
where a.status != b.status
now finding the max of the act items.
with rownumb as
(
select *, row_number() OVER (order by date asc) as rn
), statuschange as
(
select status, date
from rownumb A
join rownumb B on A.rn = B.rn-1
where a.status != b.status
)
select max(date)
from satuschange
where status='Act'