Sql query to create teams - sql-server

Question

I need a query to assign teams to a series of users. Data looks like this:
UserId Category Team
1 A null
2 A null
3 B null
4 B null
5 A null
6 B null
8 A null
9 B null
11 B null
Teams should be created by sorting by userid and the first userid becomes the team number and the consecutive A's are part of that team as are the B's that follow. The first A after the Bs starts a new team. There will always be at least one A and one B. So after the update, that data should look like this:
UserId Category Team
1 A 1
2 A 1
3 B 1
4 B 1
5 A 5
6 B 5
8 A 8
9 B 8
11 B 8
EDIT:
Need to add that the user id's will not always increment by 1. I edited the example data to show what I mean. Also, the team ID doesn't strictly have to be the id of the first user, as long as they end up grouped properly. For example, users 1 - 4 could all be on team '1', users 5 and 6 on team '2' and users 8,9 and 11 on team '3'

Answer 1

First you could label each row with an increasing number. Then you can use a left join to find the previous user. If the previous user has category 'B', and the current one category 'A', that means the start of a new team. The team number is then the last UserId that started a new team before the current UserId.
Using SQL Server 2008 syntax:
; with numbered as
(
select row_number() over (order by UserId) rn
, *
from Table1
)
, changes as
(
select cur.UserId
, case
when prev.Category = 'B' and cur.Category = 'A' then cur.UserId
when prev.Category is null then cur.UserId
end as Team
from numbered cur
left join
numbered prev
on cur.rn = prev.rn + 1
)
update t1
set Team = team.Team
from Table1 t1
outer apply
(
select top 1 c.Team
from changes c
where c.UserId <= t1.UserId
and c.Team is not null
order by
c.UserId desc
) as team;
Example at SQL Fiddle.

Answer 2

You can do this with a recursive CTE:
with userCTE as
(
select UserId
, Category
, Team = UserId
from users where UserId = 1
union all
select users.UserId
, users.Category
, Team = case when users.Category = 'A' and userCTE.Category = 'B' then users.UserId else userCTE.Team end
from userCTE
inner join users on users.UserId = userCTE.UserId + 1
)
update users
set Team = userCTE.Team
from users
inner join userCTE on users.UserId = userCTE.UserId
option (maxrecursion 0)
SQL Fiddle demo.
Edit:
You can update the CTE to get this to go:
with userOrder as
(
select *
, userRank = row_number() over (order by userId)
from users
)
, userCTE as
(
select UserId
, Category
, Team = UserId
, userRank
from userOrder where UserId = (select min(UserId) from users)
union all
select users.UserId
, users.Category
, Team = case when users.Category = 'A' and userCTE.Category = 'B' then users.UserId else userCTE.Team end
, users.userRank
from userCTE
inner join userOrder users on users.userRank = userCTE.userRank + 1
)
update users
set Team = userCTE.Team
from users
inner join userCTE on users.UserId = userCTE.UserId
option (maxrecursion 0)
SQL Fiddle demo.
Edit:
For larger datasets you'll need to add the maxrecursion query hint; I've edited the previous queries to show this. From Books Online:
Specifies the maximum number of recursions allowed for this query.
number is a nonnegative integer between 0 and 32767. When 0 is
specified, no limit is applied.
In this case I've set it to 0, i.e. not limit on recursion.
Query Hints.

Answer 3

I actually ended up going with the following. It finished on all 3 million+ rows in a half an hour.
declare #userid int
declare #team int
declare #category char(1)
declare #lastcategory char(1)
set #userid = 1
set #lastcategory='B'
set #team=0
while #userid is not null
begin
select #category = category from users where userid = #userid
if #category = 'A' and #lastcategory = 'B'
begin
set #team = #userid
end
update users set team = #team where userid = #userid
set #lastcategory = #category
select #userid = MIN(userid) from users where userid > #userid
End