Question

Suppose I have this code

create temporary table somedata (a integer); insert into somedata (a) values (11), (25), (62); --#these values are always increasing select * from somedata; 

giving this

+--+ |a | +--+ |11| |25| |62| +--+ 

How do I calculate a column of values ‘b’ where each one is the difference between the value of ‘a’ in the current row and the value of ‘a’ in the preceding row?

+--+--+ |a |b | +--+--+ |11| 0| # ie 11-11 since theres no preceding row |25|14| # ie 25-11 |62|37| # ie 62-25 etc +--+--+ 

This is so obvious in openoffice or excel that I feel a bit silly not having yet found how to do this on MySql’s site nor anywhere else.

Answer 1

It would easier if you had an auto_increment column to give each row its own id – then you could join the table against itself on (alias2.id=alias1.id+1) and calculate the diff from alias2.a-alias1.a

As it is, I think the only way is with a subquery to obtain the largest value a less than the current row value, which will be very inefficient.

So if you can, modify your schema!

create table somedata (     id int auto_increment not null,      a integer,      primary key(id) ); insert into somedata (a) values (11), (25), (62);   select a2.a,ifnull(a2.a-a1.a, 0) as diff   from somedata  as a1  right join somedata  as a2 on (a2.id=a1.id+1);  +------+------+ | a    | diff | +------+------+ |   11 |    0 | |   25 |   14 | |   62 |   37 | +------+------+ 

Use inner join rather a right join if you don’t want that first zero result.

Edit: see this article for a fuller walkthrough of this idea: Calculating differences between successive rows