--классический ретеншн
select @day 
--число типов на сл день рег
, (cast 
(
select count(*)
from dailyusers
where [day] = dateadd(day, 1, @day)
and uid in (select uid from users where dateadd( day, datediff(day, 0,reg_time), 0) = @day) 
) as float)
/
--число типов в день рег
(cast 
(select count(*)
from users
where dateadd( day, datediff(day, 0,reg_time), 0) =  @day)
)
as float)

--роллинг ретеншн
select @day 
--число типов на сл день рег и пожже
,(cast 
(
select count(distinct uid)
from dailyusers
where [day] >= dateadd(day, 1, @day)
and uid in (select uid from users where dateadd( day, datediff(day, 0,reg_time), 0) = @day) 
) as float)
/
--число типов в день рег
(cast 
(select count(*)
from users
where dateadd( day, datediff(day, 0,reg_time), 0) =  @day)
)
as float)

declare @var_date datetime;
set @var_date = '20170301';
select * from mytable where mydate = @var_date;