Grouping and counting rows by value until it changes

Question

I have a table where messages are stored as they happen. Usually there is a message \'A\' and sometimes the A\'s are separated by a single message \'B\'. Now I want to group

Answer 1

Here is a little bit smaller solution:

DECLARE @t TABLE ( d DATE, m CHAR(1) )

INSERT  INTO @t
VALUES  ( '20150301', 'A' ),
        ( '20150302', 'A' ),
        ( '20150303', 'B' ),
        ( '20150304', 'A' ),
        ( '20150305', 'A' ),
        ( '20150306', 'A' ),
        ( '20150307', 'B' );

WITH 
c1 AS(SELECT d, m, IIF(LAG(m, 1, m) OVER(ORDER BY d) = m, 0, 1) AS n FROM @t),
c2 AS(SELECT m, SUM(n) OVER(ORDER BY d) AS n FROM c1) 
    SELECT m, COUNT(*) AS c
    FROM c2
    GROUP BY m, n

Output:

m   c
A   2
B   1
A   3
B   1

The idea is to get value 1 at rows where message is changed:

2015-03-01  A   0
2015-03-02  A   0
2015-03-03  B   1
2015-03-04  A   1
2015-03-05  A   0
2015-03-06  A   0
2015-03-07  B   1

The second step is just sum of current row value + all preceding values:

2015-03-01  A   0
2015-03-02  A   0
2015-03-03  B   1
2015-03-04  A   2
2015-03-05  A   2
2015-03-06  A   2
2015-03-07  B   3

This way you get grouping sets by message column and calculated column.