mysql: 比较同一张表的相邻两行的某一字段的值

来源：http://www.iteye.com/topic/322307

觉得这个问题比较有趣，发出来给有用的人参考： 

/*Column Information For - test.tt*/ 
------------------------------------ 

Field   Type     Collation  Null    Key     Default  Extra   Privileges                       Comment 
------  -------  ---------  ------  ------  -------  ------  -------------------------------  ------- 
id      int(11)  (NULL)     YES             (NULL)           select,insert,update,references         
value   int(11)  (NULL)     YES             (NULL)           select,insert,update,references       

SQL： 
select a.id, a.value - b.value as diff from 
(select id ,value, @num := @num + 1 as row_number from (SELECT @num:=0) r,tt order by id) a, 
(select id ,value, @num2 := @num2 + 1 as row_number from (SELECT @num2:=0) r2, tt order by id) b 
where a.row_number + 1 = b.row_number ;