sql group by only rows which are in sequence - sql

Question

Say I have the following table:
MyTable
---------
| 1 | A |
| 2 | A |
| 3 | A |
| 4 | B |
| 5 | B |
| 6 | B |
| 7 | A |
| 8 | A |
---------
I need the sql query to output the following:
---------
| 3 | A |
| 3 | B |
| 2 | A |
---------
Basically I'm doing a group by but only for rows which are together in the sequence. Any ideas?
Note that the database is on sql server 2008. There is a post on this topic however it uses oracle's lag() function.

Answer 1

This is known as the "islands" problem. Using Itzik Ben Gan's approach:
;WITH YourTable AS
(
SELECT 1 AS N, 'A' AS C UNION ALL
SELECT 2 AS N, 'A' AS C UNION ALL
SELECT 3 AS N, 'A' AS C UNION ALL
SELECT 4 AS N, 'B' AS C UNION ALL
SELECT 5 AS N, 'B' AS C UNION ALL
SELECT 6 AS N, 'B' AS C UNION ALL
SELECT 7 AS N, 'A' AS C UNION ALL
SELECT 8 AS N, 'A' AS C
),
T
AS (SELECT N,
C,
DENSE_RANK() OVER (ORDER BY N) -
DENSE_RANK() OVER (PARTITION BY C ORDER BY N) AS Grp
FROM YourTable)
SELECT COUNT(*),
C
FROM T
GROUP BY C,
Grp
ORDER BY MIN(N)

Answer 2

this will work for you...
SELECT
Total=COUNT(*), C
FROM
(
SELECT
NGroup = ROW_NUMBER() OVER (ORDER BY N) - ROW_NUMBER() OVER (PARTITION BY C ORDER BY N),
N,
C
FROM MyTable
)RegroupedTable
GROUP BY C,NGroup

Answer 3

Just for fun, without any SQL-specific functions and NOT assuming that the ID column is monotonically increasing:
WITH starters(name, minid, maxid) AS (
SELECT
a.name, MIN(a.id), MAX(a.id)
FROM
mytable a RIGHT JOIN
mytable b ON
(a.name <> b.name AND a.id < b.id)
WHERE
a.id IS NOT NULL
GROUP BY
a.name
),
both(name, minid, maxid) AS (
SELECT
name, minid, maxid
FROM
starters
UNION ALL
SELECT
name, MIN(id), MAX(id)
FROM
mytable
WHERE
id > (SELECT MAX(maxid) from starters)
GROUP BY
name
)
SELECT
COUNT(*), m.name, minid
FROM
both INNER JOIN
mytable m ON
id BETWEEN minid AND maxid
GROUP BY
m.name, minid
Result (ignore the midid column):
(No column name) name minid
3 A 1
3 B 4
2 A 7