Question

I have a weird update query to write.

Here’s the table

PK-ID (int) --- FK-ID (int) --- Value (int)

In my data set, if I group by FK-ID and order by PK-ID, suppose this is an example of one group:

5 --- 10 --- 23
7 --- 10 --- 49
8 --- 10 --- 81

Due to a bug in some old software, records 7 and 8 have incorrect values. The correct value for 7 is (49-23) = 26 and the correct value for 8 is (81-49) = 32. Record 5 is correct.

I need to update each record to subtract the value of the record immediately preceding it when it is grouped by FK-ID and ordered by PK-ID. If there is no preceding record I do not need to change the value.

Is there a way to write a general sql update query to accomplish this? How would I (conditionally) retrieve the value of the preceding record in the group? I’m using SQL server 2008.

Thanks!

Answer 1

with ordered as (
    select *, rn = row_number() over (partition by fk_id order by pk_id)
      from tbl
    )
update cur
   set value = cur.value - prior.value
  from ordered cur
  join ordered prior on prior.fk_id = cur.fk_id
                and prior.rn = cur.rn-1;