mysql - Sql to find timediff between two rows based on ID -

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the subject of question not explanatory, sorry that.

ya question follows: have database structure below pk primary key, id multiple many rows.

+------+------+---------------------+ | pk   | id   | value               | +------+------+---------------------+ |   99 |    1 | 2013-08-06 11:10:00 | |  100 |    1 | 2013-08-06 11:15:00 | |  101 |    1 | 2013-08-06 11:20:00 | |  102 |    1 | 2013-08-06 11:25:00 | |  103 |    2 | 2013-08-06 15:10:00 | |  104 |    2 | 2013-08-06 15:15:00 | |  105 |    2 | 2013-08-06 15:20:00 | +------+------+---------------------+

what need is, value difference between first 2 rows (which ordered value) each group (where group id). according above structure need timediff(value100, value99) [ id 1 group] , timediff(value104, value103) [ id 2 group]

i.e. value difference of time ordered value 1st 2 rows in each group.

one way can think 3 self joins (or 3 sub queries) find first 2 in 2 of them , , third query subtracting it. suggestions?

looks bit wierd... can try way 

set @previous = 0; set @temp = 0; set @tempid = 0;

above step may not needed .. make sure nothing goes wrong

select pkid, id, diff, valtemp ( select if(@previousid = id, @temp := @temp + 1, @temp := 1) occ, @previousid := id, timediff(`value`, @previous) diff, pk, id, `value`, @previous := `value`  testtable) occ = 2

demo on sql fiddle


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