Select continuous ranges from table

Question

I need to extract continous ranges from a table based on consecutive numbers (column N) and same "category" these numbers relate to (column C below). Graphically it looks like this:

 N  C  D
--------
 1  x  a           C  N1  N2  D1  D2
 2  x  b          ------------------
 3  x  c           x   1   4   a   d     (continuous range with same N)
 4  x  d    ==>    x   6   7   e   f     (new range because "5" is missing)
 6  x  e           y   8  10   g   h     (new range because C changed to "y")
 7  x  f
 8  y  g
 9  y  h
10  y  i

SQL Server is 2005. Thanks.

Answer 1

DECLARE @myTable Table
(
    N INT,
    C CHAR(1),
    D CHAR(1)
)
INSERT INTO @myTable(N,C,D) VALUES(1,  'x', 'a');
INSERT INTO @myTable(N,C,D) VALUES(2,  'x', 'b');
INSERT INTO @myTable(N,C,D) VALUES(3,  'x', 'c');
INSERT INTO @myTable(N,C,D) VALUES(4,  'x', 'd');
INSERT INTO @myTable(N,C,D) VALUES(6,  'x', 'e');
INSERT INTO @myTable(N,C,D) VALUES(7,  'x', 'f');
INSERT INTO @myTable(N,C,D) VALUES(8,  'y', 'g');
INSERT INTO @myTable(N,C,D) VALUES(9,  'y', 'h');
INSERT INTO @myTable(N,C,D) VALUES(10, 'y', 'i');


WITH StartingPoints AS(

    SELECT A.*, ROW_NUMBER() OVER(ORDER BY A.N) AS rownum
    FROM @myTable AS A
    WHERE NOT EXISTS(
        SELECT *
        FROM @myTable B
        WHERE B.C = A.C
          AND B.N = A.N - 1
    )
 ),
 EndingPoints AS(
    SELECT A.*, ROW_NUMBER() OVER(ORDER BY A.N) AS rownum
    FROM @myTable AS A
    WHERE NOT EXISTS (
        SELECT *
        FROM @myTable B
        WHERE B.C = A.C
          AND B.N = A.N + 1
    )
 ) 
SELECT StartingPoints.C,
       StartingPoints.N AS [N1],
       EndingPoints.N AS [N2],
       StartingPoints.D AS [D1],
       EndingPoints.D AS [D2] 
FROM StartingPoints
JOIN EndingPoints ON StartingPoints.rownum = EndingPoints.rownum

Results:

C    N1          N2          D1   D2
---- ----------- ----------- ---- ----
x    1           4           a    d
x    6           7           e    f
y    8           10          g    i

Answer 2

The RANK function is a safer bet than ROW_NUMBER, in case any N values are duplicated, as in the following example:

declare @ncd table(N int, C char, D char);

insert into @ncd
select 1,'x','a' union all
select 2,'x','b' union all
select 3,'x','c' union all
select 4,'x','d' union all
select 4,'x','e' union all
select 7,'x','f' union all
select 8,'y','g' union all
select 9,'y','h' union all
select 10,'y','i' union all
select 10,'y','j';

with a as (
    select *
    , r = N-rank()over(partition by C order by N)
    from @ncd
)
select C=MIN(C)
, N1=MIN(N)
, N2=MAX(N)
, D1=MIN(D)
, D2=MAX(D)
from a
group by r;

Result, which correctly withstands the duplicated 4 and 10:

C    N1          N2          D1   D2
---- ----------- ----------- ---- ----
x    1           4           a    e
x    7           7           f    f
y    8           10          g    j