Subtracting one row of data from another in SQL

Question

I\'ve been stumped with some SQL where I\'ve got several rows of data, and I want to subtract a row from the previous row and have it repeat all the way down.

So her

Answer 1

Yipee!!! this does the trick:

SELECT  f.id, f.length, 
    (f.length - ISNULL(f2.length,0)) AS diff
FROM foo f
LEFT OUTER JOIN foo f2
ON  f2.id = (f.id +1)

Please check for other cases also, it is working for the values you posted! Note this is for SQL Server 2005

Answer 2

What about something like this:

SELECT T2.ID, T2.[Length], T2.[Length]-T1.[Length] AS 'Difference'
FROM Foo AS T1 RIGHT OUTER JOIN Foo AS T2 ON ( T1.ID = (T2.ID-1) )
ORDER BY T1.ID

Answer 3

So they are just ordered largest to smallest?

SELECT f.id, f.length, (f.length - ISNULL(t.length, 0)) AS difference
FROM foo AS f
LEFT JOIN (
    SELECT f1.id
        ,MAX(f2.length) as length
    FROM foo AS f1
    INNER JOIN foo AS f2
        ON f1.length > f2.length
    GROUP BY f1.id
) AS t -- this is the triangle
    ON t.id = f.id

You can use COALESCE (or IFNULL) instead of ISNULL for MySQL.

Answer 4

Select f1.id, f1.seqnum, f2.seqnum, f1.length, f2.length, f1.length-f2.length 

From (

Select Id, length, row_number(order by length) 'seqnum'
From
foo

) f1

Inner join (

Select 
Id, length, row_number(order by length) 'seqnum' from foo union select 0, 0, 0

) f2 

On f1.seqnum = f2.seqnum + 1

Order by f1.length desc

Answer 5

edit: fixed when re-read Q (misunderstood)

SELECT f.id, 
       f2.id, 
       f.length, 
       f2.length, 
       (f.length -f2.length) AS difference
FROM foo f, 
     foo f2 
where f2.id = f.id+1

id was ambiguous

edit: note: tested in mysql 5.0

Answer 6

I had this problem and it was interesting to look at your solutions. I find it strange that such a normal-life problem is so complicated in SQL. As I need the values in a report only, I chose a completely different solution. I'm running Ruby on Rails as the front end of my sqlite3 database, and just did the subtraction in the view like this:

In your ruby controller, there is a object variable @foo that holds the rows returned by your query.

In the view, just do

<table border=1>
  <tr>
    <th>id</th>
    <th>length</th>
    <th>difference</th>
  </tr>

<% for i in 0..@foo.length-1 do %>
  <tr>
    <td><%=h @foo[i].id %></td>
    <td><%=h @foo[i].length %></td>
    <td><% if (i==@foo.length-1) then %>
        <%=  @foo[i].length %>
      <% else %>
        <%= @foo[i+1].length.to_i - @foo[i].length.to_i %>
      <% end %>
    </td>
  </tr>
<% end %>
</table>

Seems to be more robust than the SQL solutions.