From a table of made up of rows of dates, how to count the number of rows where the date is in a certain date range

Question

I have a table dbo.MyDates:

date
--------
'08/28/2012'
'01/10/2013'
'02/05/2013'

and a table dbo.People:

id     name      dateRangeStart    dateRangeEnd
---    ------    --------------    ------------
100    Mike      '08/01/2012'      '11/15/2012'
101    John      '08/01/2012'      '02/01/2013'
102    Claire    '12/01/2012       '03/15/2013'
103    Mary      '03/01/2013'      '05/01/2013'

What I'm trying to do is check if each of the dates in rows 1 -3 are within a specific date range then total the number of dates that are within the range:

id     name      totalDaysWithinRange
---    ------    --------------------
100    Mike      1
101    John      2
102    Claire    2
103    Mary      0

so that I can use the totalDaysWithinRange in a simple math calculation. I know how to use while loops from other languages like Java and PHP, but based on my research it seems that in T-SQL it's better to use a recursive CTE. I've used CTEs in the past so I know how they work but I've never used a recursive CTE before. Here's what I came up with:

WITH cte AS 
(
  SELECT
    p.id AS personID,
    p.name AS personName,
    p.dateRangeStart AS drs,
    p.dateRangeEnd AS dre,
    d.date AS checkedDate

  FROM dbo.MyDates AS d, dbo.People AS p
  WHERE d.date BETWEEN p.dateRangeStart AND p.dateRangeEnd
  UNION ALL
  SELECT
    cte.personID,
    cte.personName,
    cte.drs,
    cte.dre,
    cte.checkedDAte
    FROM cte
      INNER JOIN dbo.MyDates AS d
      ON d.date = cte.checkedDate   
)

SELECT
  p.id
  p.name,
  COUNT(cte.personID)
  FROM cte AS c
    INNER JOIN dbo.Person AS p ON p.id = c.personID)
;

What I can't figure out is how to calculate the sums so that I can use them in a SELECT from cte. Thanks for your help.

Answer 1

This should work:

;with cte as 
(
  select id, count(*) as dayCount
  from People p
  join MyDates d on d.[Date] between p.dateRangeStart and p.dateRangeEnd
  group by id
)
select p.id, p.name, isnull(cte.dayCount,0) as totalDaysWithinRange
from cte 
right join People p on p.id = cte.id
order by p.id