How to get a value from previous result row of a SELECT statement? - sql

Question

If we have a table called FollowUp and has rows [ ID(int) , Value(Money) ]
and we have some rows in it, for example
ID --Value
1------70
2------100
3------150
8------200
20-----250
45-----280
and we want to make one SQL Query that get each row ID,Value and the previous Row Value in which data appear as follow
ID --- Value ---Prev_Value
1 ----- 70 ---------- 0
2 ----- 100 -------- 70
3 ----- 150 -------- 100
8 ----- 200 -------- 150
20 ---- 250 -------- 200
45 ---- 280 -------- 250
i make the following query but i think it's so bad in performance in huge amount of data
SELECT FollowUp.ID, FollowUp.Value,
(
SELECT F1.Value
FROM FollowUp as F1 where
F1.ID =
(
SELECT Max(F2.ID)
FROM FollowUp as F2 where F2.ID < FollowUp.ID
)
) AS Prev_Value
FROM FollowUp
So can anyone help me to get the best solution for such a problem ?

Answer 1

This sql should perform better then the one you have above, although these type of queries tend to be a little performance intensive... so anything you can put in them to limit the size of the dataset you are looking at will help tremendously. For example if you are looking at a specific date range, put that in.
SELECT followup.value,
( SELECT TOP 1 f1.VALUE
FROM followup as f1
WHERE f1.id<followup.id
ORDER BY f1.id DESC
) AS Prev_Value
FROM followup
HTH

Answer 2

You can use the OVER statement to generate nicely increasing row numbers.
select
rownr = row_number() over (order by id)
, value
from your_table
With the numbers, you can easily look up the previous row:
with numbered_rows
as (
select
rownr = row_number() over (order by id)
, value
from your_table
)
select
cur.value
, IsNull(prev.value,0)
from numbered_rows cur
left join numbered_rows prev on cur.rownr = prev.rownr + 1
Hope this is useful.

Answer 3

This is not an answer to your actual question.
Instead, I feel that you are approaching the problem from a wrong direction:
In properly normalized relational databases the tuples ("rows") of each table should contain references to other db items instead of the actual values. Maintaining these relations between tuples belongs to the data insertion part of the codebase.
That is, if containing the value of a tuple with closest, smaller id number really belongs into your data model.
If the requirement to know the previous value comes from the view part of the application - that is, a single view into the data that needs to format it in certain way - you should pull the contents out, sorted by id, and handle the requirement in view specific code.
In your case, I would assume that knowing the previous tuples' value really would belong in the view code instead of the database.
EDIT: You did mention that you store them separately and just want to make a query for it. Even still, application code would probably be the more logical place to do this combining.

Answer 4

What about pulling the lines into your application and computing the previous value there?

Answer 5

Create a stored procedure and use a cursor to iterate and produce rows.

Answer 6

You could use the function 'LAG'.
SELECT ID,
Value,
LAG(value) OVER(ORDER BY ID) AS Prev_Value
FROM FOLLOWUP;