Using SQL to get the previous rows data - sql-server-2005

Question

I have a requirement where I need to get data from the previous row to use in a calculation to give a status to the current row. It's a history table. The previous row will let me know if a data has changed in a date field.
I've looked up using cursors and it seems a little complicated. Is this the best way to go?
I've also tried to assgin a value to a new field...
newField =(Select field1 from Table1 where "previous row") previous row is where I seem to get stuck. I can't figure out how to select the row beneath the current row.
I'm using SQL Server 2005
Thanks in advance.

Answer 1

-- Test data
declare #T table (ProjectNumber int, DateChanged datetime, Value int)
insert into #T
select 1, '2001-01-01', 1 union all
select 1, '2001-01-02', 1 union all
select 1, '2001-01-03', 3 union all
select 1, '2001-01-04', 3 union all
select 1, '2001-01-05', 4 union all
select 2, '2001-01-01', 1 union all
select 2, '2001-01-02', 2
-- Get CurrentValue and PreviousValue with a Changed column
;with cte as
(
select *,
row_number() over(partition by ProjectNumber order by DateChanged) as rn
from #T
)
select
C.ProjectNumber,
C.Value as CurrentValue,
P.Value as PreviousValue,
case C.Value when P.Value then 0 else 1 end as Changed
from cte as C
inner join cte as P
on C.ProjectNumber = P.ProjectNumber and
C.rn = P.rn + 1
-- Count the number of changes per project
;with cte as
(
select *,
row_number() over(partition by ProjectNumber order by DateChanged) as rn
from #T
)
select
C.ProjectNumber,
sum(case C.Value when P.Value then 0 else 1 end) as ChangeCount
from cte as C
inner join cte as P
on C.ProjectNumber = P.ProjectNumber and
C.rn = P.rn + 1
group by C.ProjectNumber

Answer 2

This really depends on what tells you a row is a "Previous Row". however, a self join should do what you want:
select *
from Table1 this
join Table2 prev on this.incrementalID = prev.incrementalID+1

Answer 3

If you have the following table
CREATE TABLE MyTable (
Id INT NOT NULL,
ChangeDate DATETIME NOT NULL,
.
.
.
)
The following query will return the previous record for any record from MyTable.
SELECT tbl.Id,
tbl.ChangeDate,
hist.Id,
hist.ChangeDate
FROM MyTable tbl
INNER JOIN MyTable hist
ON hist.Id = tbl.Id
AND hiost.ChangeDate = (SELECT MAX(ChangeDate)
FROM MyTable sub
WHERE sub.Id = tbl.Id AND sub.ChangeDate < tbl.ChangeDate)