Total number of days for a task before going on to the next one, grouped by person - sql

Question

I am trying to figure out how to show how many days have been worked on a certain task by using the dates in between each “task login” for each person. I think this can be done with one query? I'm open to suggestions and/or ideas.
The Table:
--------+-----------+----------
Person | TaskLogin | Date
--------+-----------+----------
Jane | A | 2013-01-01
Jane | B | 2013-01-03
Jane | A | 2013-01-06
Jane | B | 2013-01-10
Bob | A | 2013-01-01
Bob | A | 2013-01-06
---------------------------------------------------------------------
Row 1: Jane starts task A starting 2013-01-01 and works on it until starting Task B on 2013-01-03 = 2 days worked on Task A
Row 2: Jane starts on task B starting 2013-01-03 and works on it until starting task A on 2013-01-06 = 3 days worked on Task B
Row 3: Jane starts on task A starting 2013-01-06 and works on it until starting task B on 2013-01-10 = 4 days worked on Task A
Row 4: Skip because that is the highest date for Jane (Jane may or may not finish task B 2013-01-10 but we will not count it)
Row 5: Bob starts task A starting on 2013-01-01 and works on it until continuing to work on task A by logging it again on 2013-01-06 = 5 days worked on task A
Row 6: Skip because that is the highest date for Bob
A = 11 days because 2 + 4 + 5
B = 3 days because of Row 2
The output:
------+---------------------
Tasks | Time between Tasks
------+---------------------
A | 11 days
B | 3 days
**EDIT:*****
The solutions of Nicarus and Gordon Linoff (first pre-2013 solution specifically, with my edits in the comments) works. Note that (select distinct * from table t) t for table can be added to Gordon Linoff's solution to accommodate for the case of someone logging in twice in the same day.

Answer 1

What you are looking for is the lead() function. This is only available in SQL Server 2012. Before that, the easiest way is a correlated subquery:
select TaskLogin, sum(datediff(day, date, nextdate)) as days
from (select t.*,
(select top 1 date
from table t2
where t2.person = t.person
order by date desc
) as nextdate
from table t
) t
where nextdate is not null
group by TaskLogin;
In SQL Server 2012, it would be:
select TaskLogin, sum(datediff(day, date, nextdate)) as days
from (select t.*, lead(date) over (partition by person order by date) as nextdate
from table t
) t
where nextdate is not null
group by TaskLogin;

Answer 2

Maybe not the most elegant way, but it certainly works:
-- Setup table/insert values --
IF OBJECT_ID('TempDB.dbo.#TaskAccounting') IS NOT NULL BEGIN
DROP TABLE #TaskAccounting
END
CREATE TABLE #TaskAccounting
(
Person VARCHAR(4) NOT NULL,
TaskLogin CHAR(1) NOT NULL,
TaskDate DATETIME NOT NULL
)
INSERT INTO #TaskAccounting
VALUES ('Jane','A','2013-01-01')
INSERT INTO #TaskAccounting
VALUES ('Jane','B','2013-01-03')
INSERT INTO #TaskAccounting
VALUES ('Jane','A','2013-01-06')
INSERT INTO #TaskAccounting
VALUES ('Jane','B','2013-01-10')
INSERT INTO #TaskAccounting
VALUES ('Bob','A','2013-01-01')
INSERT INTO #TaskAccounting
VALUES ('Bob','A','2013-01-06');
-- Use a CTE to add sequence and join on it --
WITH Tasks AS (
SELECT
Person,
TaskLogin,
TaskDate,
ROW_NUMBER() OVER(PARTITION BY Person ORDER BY TaskDate) AS Sequence
FROM
#TaskAccounting
)
SELECT
a.TaskLogin AS Tasks,
CAST(SUM(DATEDIFF(DD,a.TaskDate,b.TaskDate)) AS VARCHAR) + ' days' AS TimeBetweenTasks
FROM
Tasks a
JOIN
Tasks b
ON (a.Person = b.Person)
AND (a.Sequence = b.Sequence - 1)
GROUP BY
a.TaskLogin