SQL Query for Grouping the results based on sequence

Question

I have a table like this:

ID  Seq  Amt
1   1    500
1   2    500
1   3    500
1   5    500
2   10   600
2   11   600
3   1    700
3   3    700

Answer 1

This seems to work nicely. @breakingRows will contain all rows that break the sequence of id and seq (i.e. if id changes or if seq is not 1 more than the previous seq). With that table you can select all rows of such a sequence within @temp. I must add however that performance will probably be not all that good because of all the subqueries but you'll need to test to be sure.

declare @temp table (id int, seq int, amt int)
insert into @temp select 1, 1, 500
insert into @temp select 1, 2, 500
insert into @temp select 1, 3, 500
insert into @temp select 1, 5, 500
insert into @temp select 2, 10, 600
insert into @temp select 2, 11, 600
insert into @temp select 3, 1, 700
insert into @temp select 3, 3, 700

declare @breakingRows table (ctr int identity(1,1), id int, seq int)

insert into @breakingRows(id, seq)
select id, seq
from @temp t1 
where not exists 
    (select 1 from @temp t2 where t1.id = t2.id and t1.seq - 1 = t2.seq)
order by id, seq

select br.id, br.seq as start, 
       isnull ((select top 1 seq from @temp t2 
               where id < (select id from @breakingRows br2 where br.ctr = br2.ctr - 1) or 
                     (id = (select id from @breakingRows br2 where br.ctr = br2.ctr - 1) and
                      seq < (select seq from @breakingRows br2 where br.ctr = br2.ctr - 1))          
               order by id desc, seq desc),
               br.seq)
      as [end],
      (select SUM(amt) from @temp t1 where t1.id = br.id and 
        t1.seq < 
            isnull((select seq from @breakingRows br2 where br.ctr = br2.ctr - 1 and br.id = br2.id), 
                   (select max(seq) + 1 from @temp)) and 
        t1.seq >= br.seq)
from @breakingRows br
order by id, seq