T-SQL multiple datetime diff on one line - sql

Question

Using SQL Server (T-SQL), and I have two tables:
tblTrial:
TrialID (PK) int
TrialDate
...
tblLaps:
LapID (PK) int
TrialID (FK) int
LapNumber int
LapStart smalldatetime
...
For TrialID = 1, there are four Lap rows:
LapID TrialID LapNumber LapStart
1 1 1 t1 (some smalldatetime value)
2 1 2 t2
3 1 3 t3
4 1 4 t4
I want to display the SQL so that for each Trial, only one row is displayed, and it has the time differences.
For example, a row for TrialID = 1 might look like:
Trial# 1stLap 2ndLap 3rdLap
---------------------------------
1 3min 4min 5min
where 1stLap is time difference t2-t1, 2ndLap is t3-t2, 3rdLap is t4-t3.
How do I make everything go on one line in a SQL statement?
Thanks

Answer 1

select
t.TrialID
, datediff(minute, max(case when LapNumber = 1 then LapStart end) , max(case when LapNumber = 2 then LapStart end) ) lap1_2
, datediff(minute, max(case when LapNumber = 2 then LapStart end) , max(case when LapNumber = 3 then LapStart end) ) lap2_3
, datediff(minute, max(case when LapNumber = 3 then LapStart end) , max(case when LapNumber = 4 then LapStart end) ) lap3_4
from tblTrial as t
inner join tblLaps as l on t.TrialID = l.TrialID
group by
t.TrialID
see this sqlfiddle demo

Answer 2

For time difference in minutes, you could do like this:
with laps as -- First CTE table to join every lap with the next lap to get end time
(
select TrialID,t1.LapID, datediff(mi,t1.LapStart, t2.LapStart ) as Lap
from tblLaps t1
join tblLaps t2
on t1.LapID = t2.LapID - 1 and t1.TrialID = t2.TrialID
)
select TrialID,
max(case t1.LapID when 1 then Lap else null end) as [1stLap],
max(case t1.LapID when 2 then Lap else null end) as [2ndLap],
max(case t1.LapID when 3 then Lap else null end) as [3rdLap],
from laps
group by TrialID
If you want differ by seconds, use datediff(ss,startdate , enddate ), here's the datediff document.