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

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

<% for i in 0..@foo.length-1 do %>
  
<% end %>

  

    
id
    
length
    
difference
  

    
<%=h @foo[i].id %>
    
<%=h @foo[i].length %>
    
<% if (i==@foo.length-1) then %>
        <%=  @foo[i].length %>
      <% else %>
        <%= @foo[i+1].length.to_i - @foo[i].length.to_i %>
      <% end %>
    
  

Seems to be more robust than the SQL solutions.