Question

Suppose I have a table with columns (DayId, RunningTotal):

DayId    RunningTotal
---------------------
1        25
3        50
6        100
9        200
10       250

How can I select the DayId and the amount the RunningTotal has increased from the previous day? i.e. how can I select:

DayId    DayTotal
---------------------
1        25
3        25
6        50
9        100
10       50

The only current method I know is with a while loop I am trying to factor out. Also, the DayId has no regular rules, just that it is some increasing integer value, but it increases by an irregular amount as shown in the example table.

EDIT: using MS SQL Server 2005

Answer 1

with cte as (
  select dayid, runningtotal, row_number() over (order by dayid asc) as row_index
  from #the_table
)
select cur.dayid, cur.runningtotal - coalesce(prev.runningtotal, 0) as daytotal
from cte cur
     left join cte prev on prev.row_index = cur.row_index - 1

(I really wish they’d implemented support for the lead and lag functions in SQL Server :|)