grupo sql por sólo las filas que están en secuencia

Question

Decir que tengo la siguiente tabla:

MyTable 
--------- 
| 1 | A | 
| 2 | A | 
| 3 | A | 
| 4 | B | 
| 5 | B | 
| 6 | B | 
| 7 | A | 
| 8 | A | 
--------- 

necesito la consulta SQL para emitir la siguiente:

--------- 
| 3 | A | 
| 3 | B | 
| 2 | A | 
--------- 

Básicamente estoy haciendo un group by pero solo para las filas que están juntas en la secuencia. ¿Algunas ideas?

Tenga en cuenta que la base de datos se encuentra en el servidor sql 2008. Hay una publicación sobre este tema, sin embargo, utiliza la función lag() de oráculo.

Answer 1

Esto se conoce como el problema de "islas". Utilizando el enfoque de Itzik Ben Gan:

;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

esto funcionará para usted ...

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

Sólo por diversión, sin ningún tipo de funciones específicas de SQL y no asumir que la columna ID aumenta monotónicamente:

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 

Resultado (ignorar la columna midid):

(No column name) name minid 
3 A 1 
3 B 4 
2 A 7