Using SQL to get the previous rows data

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