FIRST_VALUE & LAST_VALUE

Question

Given the following sample data:
AverageRate CurrencyDate
1.0001            2001-09-03
1.0001            2001-09-04
1.0002            2001-09-05
1.0002            2001-09-06
1.0005            2001-09-07
1.0005            2001-09-08
1.0005            2001-09-09
1.0001            2001-09-10
1.0002            2001-09-11
1.0002            2001-09-12
How can I, using FIRST_VALUE and LAST_VALUE, return the desired output:
AverageRate From            To
1.0001            2001-09-03   2001-09-04
1.0002            2001-09-05   2001-09-06
1.0005            2001-09-07   2001-09-09
1.0001            2001-09-10   2001-09-10
1.0002            2001-09-11   2001-09-12
I know how to do this using min/max, self joins and row_number() with partition by, but surely with 2012 there is a simpler (and more optimized) solution which only requires one select statement.  Are my expectations too much?

Answer 1

First_Value, Lag, Lead, ROWS, and RANGE are great additions (Oracle still has more cool analytic functions than SQL server though), but they won't necessarily magically solve all problems.  If your data didn't have the unusual position related duplicates
(where 1.0002 showed up again later in the list, but is a different group to the first time through), your query might have fit 100 percent nicely into a less complex First_Value scenario.
Here's a way that takes care of it, that does use new SQL 2012 feature ROWS and also FIRST_VALUE.  The key element is the "ROWS" clause, where we were able to limit the window to rows that occurred BEFORE the current row.  Then FIRST_VALUE
at the final select statement too.
Create_Testdata:
Declare @Tbl table (avgrate decimal(9,4), currdate date)
Insert @Tbl Select 1.0001, '2001-09-03'
Insert @Tbl Select 1.0001, '2001-09-04'
Insert @Tbl Select 1.0002, '2001-09-05'
Insert @Tbl Select 1.0002, '2001-09-06'
Insert @Tbl Select 1.0005, '2001-09-07'
Insert @Tbl Select 1.0005, '2001-09-08'
Insert @Tbl Select 1.0005, '2001-09-09'
Insert @Tbl Select 1.0001, '2001-09-10'
Insert @Tbl Select 1.0002, '2001-09-11'
Insert @Tbl Select 1.0002, '2001-09-12'
Lister:
With L1_RowNums_Lead_Lag as
Select *
, row_number() over(order by @@Servername) as RN
, lead(avgrate) over(order by @@servername) as NextRate
, lag(avgrate) over(order by @@servername) as PrevRate
From @Tbl
, L2_RowFamily as
Select *
, case When (Prevrate <> avgrate and NextRate <> Avgrate) then RN
when NextRate = AvgRate and IsNull(PrevRate, -1) <> AvgRate then RN
Else NULL
End as RowFamily
From L1_RowNums_Lead_Lag
, L3_RowFamily_Assign as
Select *
, case when RowFamily is Null
Then Max(RowFamily) over(partition by avgrate order by rn Rows between unbounded preceding and current row)
Else RowFamily
End as RowFamily_All
from L2_RowFamily
, L4_Final as
Select *
, first_Value(Currdate) over(partition by RowFamily_all order by RowFamily_all) as FirstCurrdate
, Last_Value(Currdate) over(partition by RowFamily_all order by RowFamily_all) as LastCurrdate
From L3_RowFamily_Assign
Select * from L4_Final /* Where clause has to be in final query */
where RowFamily is not null
order by RN;
Results match your posted desired results.