Cumulative column values from dates current and previous

Question

I have a table of registrations, around 300K records. I need a SQL statement that will show the total number of registrations for that particular day?

select
count('x'),CONVERT(varchar(12),date_created,111)
from reg group by
cONVERT(varchar(12),date_created,111)
order by
CONVERT(varchar(12),date_created,111)

Result of this query:

169      2011/03/24
3016     2011/03/25
2999     2011/03/26

Desired outcome:

 2011/03/25  3016+169
 2011/03/26  2999+3016+169

How can this be done?

Answer 1

Here is two versions to do this. I have tested with 100000 rows spread over 6000 days on a really slow computer with not enough memory, and that shows that the cte version is faster than the loop version. The other versions suggested here (so far) is a lot slower, provided that I have understood the problem correctly.

Recursive CTE (10 seconds)

-- Table variable to hold count for each day
declare @DateCount table(d int, c int, rn int)
insert into @DateCount
  select 
    datediff(d, 0, date_created) as d,
    count(*) as c,
    row_number() over(order by datediff(d, 0, date_created)) as rn
  from reg
  group by datediff(d, 0, date_created)

-- Recursive cte using @DateCount to calculate the running sum
;with DateSum as
(
  select 
    d, c, rn
  from @DateCount
  where rn = 1
  union all
  select 
    dc.d, ds.c+dc.c as c, dc.rn
  from DateSum as ds
    inner join @DateCount as dc
      on ds.rn+1 = dc.rn  
)
select
  dateadd(d, d, 0) as date_created,
  c as total_num
from DateSum
option (maxrecursion 0)

Loop (14 seconds)

-- Table variable to hold count for each day
declare @DateCount table(d int, c int, rn int, cr int)
insert into @DateCount
  select 
    datediff(d, 0, date_created) as d,
    count(*) as c,
    row_number() over(order by datediff(d, 0, date_created)) as rn,
    0
  from reg
  group by datediff(d, 0, date_created)

declare @rn int = 1

-- Update cr with running sum
update dc set
  cr = dc.c  
from @DateCount as dc
where rn = @rn

while @@rowcount = 1
begin
  set @rn = @rn + 1

  update dc set
    cr = dc.c + (select cr from @DateCount where rn = @rn - 1)  
  from @DateCount as dc
  where rn = @rn
end

-- Get the result
select
  dateadd(d, d, 0) as date_created,
  cr as total_num
from @DateCount

Edit 1 The really fast version

The quirky update

-- Table variable to hold count for each day
declare @DateCount table(d int primary key, c int, cr int)
insert into @DateCount
  select 
    datediff(d, 0, date_created) as d,
    count(*) as c,
    0
  from reg
  group by datediff(d, 0, date_created)

declare @rt int = 0
declare @anchor int

update @DateCount set
  @rt = cr = @rt + c,
  @anchor = d
option (maxdop 1)

-- Get the result
select
  dateadd(d, d, 0) as date_created,
  cr as total_num
from @DateCount                
order by d