How to merge time intervals in SQL Server

Question

Suppose I have the following an event table with personId, startDate and endDate.

I want to know how much time the person X sp

Answer 1

You can use a recursive CTE to build a list of dates and then count the distinct dates.

declare @T table
(
  startDate date,
  endDate date
);

insert into @T values
('2011-01-01', '2011-01-05'),
('2011-01-04', '2011-01-08'),
('2011-01-11', '2011-01-15');

with C as
(
  select startDate,
         endDate
  from @T
  union all
  select dateadd(day, 1, startDate),
         endDate
  from C
  where dateadd(day, 1, startDate) < endDate       
)
select count(distinct startDate) as DayCount
from C
option (MAXRECURSION 0)

Result:

DayCount
-----------
11

Or you can use a numbers table. Here I use master..spt_values:

declare @MinStartDate date
select @MinStartDate = min(startDate)
from @T

select count(distinct N.number)
from @T as T
  inner join master..spt_values as N
    on dateadd(day, N.Number, @MinStartDate) between T.startDate and dateadd(day, -1, T.endDate)
where N.type = 'P'