Question

I have a mysql table A which stores a sequence of numbers and timestamps

value    timestamp
12087    2012-01-01 00:00:00
12110    2012-01-02 02:00:00
12223    2012-01-04 03:00:00
12312    2012-01-04 05:00:00
12387    2012-01-04 06:00:00
12388    2012-01-05 01:11:23
...

The both values and timestamps are ordered: the greater timestamp the greater value, but there is no other dependency between them.

I need to find the greatest difference between values in sorted sequence for a period. In the example the differences are:

12110-12087=23
12223-12100=13
12312-12223=89
12387-12312=75
12388-12387=1

So we have the greatest difference 89.

The question is: how to code SQL query for that? Do know, that I don’t want to use full product and/or correlated queries since the table is enormous.

Answer 1

select 
    (case when (@prev is null) then 0
    else abs(value - @prev) end) as diff
  , concat(value, '-', @prev)
  , @prev := value

    from (select value, dt from table1 order by dt) ordered
    order by diff limit 1

add the period restrictors into the from subquery

edit, new info tells me this is an unreliable method
http://dev.mysql.com/doc/refman/5.0/en/user-variables.html (ctrl + f for SELECT @a, @a:=@a+1)