计算和总和日期范围内的天数

Question

I have the following table:

id | start_date  |  end_date  | client_id
1    2013-08-01    2013-08-09       1
2    2013-08-10    2013-08-10       1
3    2013-08-10    2013-08-17       1
4    2013-08-18    2013-08-18       1
5    2013-08-18    2013-08-18       1
6    2013-08-18    2013-08-31       1
7    2013-08-01    2013-08-09       2
8    2013-08-11    2013-08-11       2
9    2013-08-11    2013-08-17       2
10   2013-08-19    2013-08-20       2

what I'm trying to do is count the number of days that each client was present without repeating the days for each client, so from the previous data I'm looking to get:

client_id | total_days
    1           31
    2           18

So for client 1 I get 31 because he was "present" for 31 days, from 8/1/2013 - 8/31/2013 with no gaps, and for client 2 I get 18 because he was present for 18 days:

8/1 - 8/9 = 9 days 
8/11 - 8/17 = 7 days 
8/19 - 8/20 = 2 days

is there anyway to achieve this in MySQL, I've been trying for a while but have no idea on how to do it.

This is the fiddle

Answer 1

If overlapping ranges exist, then I suggest building a driver table that is a list of dates, then JOIN to that table using BETWEEN :

SELECT a.Client_ID, COUNT(DISTINCT b.Date)
FROM YourTable a
JOIN Dates b
  ON b.Date BETWEEN a.start_date  AND a.end_date
GROUP BY a.Client_ID

Demo: SQL Fiddle

There are plenty of places to find calendar table logic, here's one.

If ranges never overlap then you can use SUM(DATEDIFF()) .

Answer 2

If there are no overlapping ranges. You can use the following query:

SELECT Client_id,
       Sum(DATEDIFF(End_date, Start_date)) AS `Present`
FROM TABLE
GROUP BY Client_id;

This will give you an overview of the number of days a client was present.