Mathematical comparison between rows

Question

I have a table

+----+-----------+---------+
| ID | StartTime | EndTime |
+----+-----------+---------+
|  1 | 2:00pm    | 3:00pm  |
|  2 | 4:00pm    | 5:00pm  |
|  3 | 7:00pm    | 9:00pm  |
+----+-----------+---------+

I need to get the difference between the end time of one row and the start time of the NEXT row. ie End time of row 1 compared to start time of row 2, or end time of row 2 compared to start time of row 3.

Ideally I'd like a result that looks similar to

+----+----------------+
| ID | TimeDifference |
+----+----------------+
|  2 | 1.0 hours      |
|  3 | 2.0 hours      |
+----+----------------+

I have no clue whatsoever on how to do something like this. I'm thinking that I may need 2 temp tables, one to hold start times another for end times so that I can more easily do the comparisons, but honestly that's just a shot in the dark at the moment.

FYI, on server 2008 in case that makes a difference for some of the commands.

Answer 1

NOTE: The question was not tagged SQL Server 2008 when this answer was written.

You can use lag() :

select t.*,
       datediff(minute, lag(endtime) over (order by id), starttime) / 60.0 as hours_diff
from t;

This does not filter out any rows. The description of the problem ("next row") and the sample data (which is based on "previous row") are inconsistent.

Answer 2

Well, since it's 2008 version you can't use the Lead() or Lag() window functions, but you can use subqueries to mimic them:

SELECT Id, 
       DATEDIFF(minute,
            (
                SELECT TOP 1 EndTime
                FROM table t1
                WHERE t1.Id < t0.Id 
                ORDER BY t1.Id DESC
            ), StartTime) / 60.0 As TimeDifference 
FROM Table t0
WHERE EXISTS             
(
    SELECT 1 
    FROM Table t2
    WHERE t2.Id < t0.Id    
)

Answer 3

You can try it

declare @t as table (ID int,  StartTime time , EndTime time)
INSERT @t SELECT 1  ,'2:00pm',     '3:00pm'  
INSERT @t SELECT 2  ,'4:00pm',     '5:00pm'  
INSERT @t SELECT 3  ,'7:00pm',     '9:00pm'


---- For sequential IDs
select 
a.ID
,a.StartTime
,a.EndTime
,datediff(minute, (SELECT EndTime FROM @t b where b.ID = a.ID - 1) , a.StartTime) / 60.0 as hours_diff
from @t a 


---- For non-sequential IDs

;WIth cte_times as (
SELECT 
    ROW_NUMBER() OVER (ORDER BY Id) as new_ID
    , ID
    ,StartTime
    ,EndTime
FROM 
    @t
) 
select 
a.ID
,a.StartTime
,a.EndTime
,datediff(minute, (SELECT EndTime FROM cte_times b where b.new_ID = a.new_ID - 1) , a.StartTime) / 60.0 as hours_diff
from cte_times a