T-sql Reset Row number on Field Change

Question

Similar to a recent post of mine \"t-sql sequential duration\"”, but not exactly the same, I want to reset the row number based on a change in column x (in my case, column \

Answer 1

The only solution I can think of is to use a cursor (ugh) and suffer through the RBAR process. NOT an elegant solution as the cursor will have to read in excess of 1m rows. Bummer.

Answer 2

Instead of:

PARTITION BY custno ORDER BY custno, moddate, who)

try:

PARTITION BY custno, who ORDER BY custno, moddate)

Answer 3

If you are on SQL Server 2012 you can use LAG to compare value with previous row and you can use SUM and OVER to record the changes.

with C1 as
(
  select custno,
         moddate,
         who,
         lag(who) over(order by moddate) as lag_who
  from chr
),
C2 as
(
  select custno,
         moddate,
         who,
         sum(case when who = lag_who then 0 else 1 end) 
            over(order by moddate rows unbounded preceding) as change 
  from C1
)
select row_number() over(partition by change order by moddate) as RowID,
       custno,
       moddate,
       who
from C2

SQL Fiddle

Update:

A version for SQL Server 2005. It uses a recursive CTE and a temp table for intermediary storage of the data you need to iterate over.

create table #tmp
(
  id int primary key,
  custno int not null,
  moddate datetime not null,
  who varchar(10) not null
);

insert into #tmp(id, custno, moddate, who)
select row_number() over(order by moddate),
       custno,
       moddate,
       who
from chr;

with C as
(
  select 1 as rowid,
         T.id,
         T.custno,
         T.moddate,
         T.who,
         cast(null as varchar(10)) as lag_who
  from #tmp as T
  where T.id = 1
  union all
  select case when T.who = C.who then C.rowid + 1 else 1 end,
         T.id,
         T.custno,
         T.moddate,
         T.who,
         C.who
  from #tmp as T
    inner join C
      on T.id = C.id + 1
)
select rowid,
       custno,
       moddate,
       who
from C
option (maxrecursion 0);

drop table #tmp;

SQL Fiddle

Answer 4

I had success with this issue by using Rank():

SELECT RANK() OVER (PARTITION BY who ORDER BY custno, moddate) AS RANK

This returned your desired results. I actually found this post trying to solve the same problem.