row_number or rank similar data - sql

Question

I have a table like this:
col1 col2
A A
A A
A F
B B
B B
B H
C L
A A
A A
A A
A E
C C
C C
C C
C C
C C
C J
And I want result like this:
col1 count
A 3
B 3
C 1
A 4
C 6
If the col1 <> col2 reset count... But I only want sql code not pl-sql etc.
Maybe row_number() over(RESET WHEN col1<>col2).
Please help me.
Ok freinds thank you. sorry for my bad english.
In fact my table like this :
id col1 col2
1000 A A
2000 A A
3000 A F
4000 B B
5000 B B
6000 B H
7000 C L
8000 A A
9000 A A
10000 A A
11000 A E
12000 C C
13000 C C
14000 C C
15000 C C
16000 C C
17000 C J
Id column is unique and has ordered values always. Maybe this will help us to solve problem. Sorry for my missing information to you. And I want solution like above.
I only want col1 and count. But not col1 unique, count must be 1,2,3 bla bla bla... until col1 <> col2...
After this row count must be reset.

Answer 1

First, I'd like to note, that without having an ORDER BY clause, you cannot guarantee the order of the results. To do this sort of calculation, it would be useful to have an identity (auto-incremental) field to establish an order.
That said, you can attempt to use ROW_NUMBER() to create a field to order on.
with yourtablewithrn as (
select col1, col2, row_number() over (order by (select null)) rn
from yourtable
),
yourtablegrouped as (
select *,
rn - row_number() over (partition by col1 order by rn) as grp
from yourtablewithrn
)
select col1,
count(col2) AS cnt
from yourtablegrouped
group by col1, grp
order by min(rn)
SQL Fiddle Demo

Answer 2

As mentioned above I agree with sgeddes that we need some kind of order that we can rely on for this kind of problem. row_number() over () wont do since it more or less is a random number:
create table yourtable
( n int
, col1 varchar(1)
, col2 varchar(1));
insert into yourtable values
(1,'A','A'),
(2,'A','A'),
(3,'A','F'),
(4,'B','B'),
(5,'B','B'),
(6,'B','H'),
(7,'C','L'),
(8,'A','A'),
(9,'A','A'),
(10,'A','A'),
(11,'A','E'),
(12,'C','C'),
(13,'C','C'),
(14,'C','C'),
(15,'C','C'),
(16,'C','C'),
(17,'C','J');
For this sample data col2 has no impact. We could do (a slight variation of sgeddes solution):
select col1, count(1)
from (
select n, col1
, col2
, row_number() over (order by n)
- row_number() over (partition by col1
order by n) as grp
from yourtable
) t
group by col1, grp
order by min(n)
But, what should the result be with a sample like below?
delete from yourtable;
insert into yourtable values
('A','A'),
('A','A'),
('A','F'),
('A','A'),
('A','G');