怎么对非连续记录相减

如何对非连续记录相减
表1
  ABC
1a10
2b11
3b13
4b15
5c16
6a18
7c20
8b25
9a31
10c34
11b38
12a39
13a45
14a47
15c49
16c55
请教用SELECT语句如何查询字段B中，a、b、c对应的字段C值的变动

[解决办法]
是这个意思吗？

SQL code

create table tb(A int,B varchar(1),c int)insert into tb select 1, 'a', 10 union allselect 2, 'b', 11 union allselect 3, 'b', 13 union allselect 4, 'b', 15 union allselect 5, 'c', 16 union allselect 6, 'a', 18 union allselect 7, 'c', 20 union allselect 8, 'b', 25 union allselect 9, 'a', 31 union allselect 10, 'c', 34 union allselect 11, 'b', 38 union allselect 12, 'a', 39 union allselect 13, 'a', 45 union allselect 14, 'a', 47 union allselect 15, 'c', 49 union allselect 16, 'c', 55 select *,(select top 1 c from tb b where a.B =b.B and a.c<b.c order by b.B ,b.c) -a.c  from tb a
[解决办法]
需要得到的结果是什么？
[解决办法]
SQL code
if object_id('tb') is not null    drop table tbcreate table tb(A int,B varchar(1),c int)insert into tb select 1, 'a', 10 union allselect 2, 'b', 11 union allselect 3, 'b', 13 union allselect 4, 'b', 15 union allselect 5, 'c', 16 union allselect 6, 'a', 18 union allselect 7, 'c', 20 union allselect 8, 'b', 25 union allselect 9, 'a', 31 union allselect 10, 'c', 34 union allselect 11, 'b', 38 union allselect 12, 'a', 39 union allselect 13, 'a', 45 union allselect 14, 'a', 47 union allselect 15, 'c', 49 union allselect 16, 'c', 55 select a.*,     a.C - isnull(b.C,0) as deltafrom tb aleft join tb b on b.B = a.B    and b.A < a.A    and not exists (        select 1 from tb c        where c.B = a.B        and c.A < a.A        and c.A > b.A)
[解决办法]
SQL code
IF EXISTS (SELECT 1 FROM SYSOBJECTS WHERE name = 'tba')BEGIN    DROP TABLE tbaENDGOCREATE TABLE tba(    A int,    B varchar(1),    C int)GOinsert into tba select 1, 'a', 10 union allselect 2, 'b', 11 union allselect 3, 'b', 13 union allselect 4, 'b', 15 union allselect 5, 'c', 16 union allselect 6, 'a', 18 union allselect 7, 'c', 20 union allselect 8, 'b', 25 union allselect 9, 'a', 31 union allselect 10, 'c', 34 union allselect 11, 'b', 38 union allselect 12, 'a', 39 union allselect 13, 'a', 45 union allselect 14, 'a', 47 union allselect 15, 'c', 49 union allselect 16, 'c', 55SELECT A,B,C,C - ISNULL((SELECT TOP 1 C FROM tba WHERE B = t.B AND A < t.A ORDER BY A DESC),0) FROM tba AS t ORDER BY B,AA    B    C    (No column name)1    a    10    106    a    18    89    a    31    1312    a    39    813    a    45    614    a    47    22    b    11    113    b    13    24    b    15    28    b    25    1011    b    38    135    c    16    167    c    20    410    c    34    1415    c    49    1516    c    55    6